[opencv4:android]: Correct paths in portfile

[pthom]

Describe the pull request

This PR handles installation issues for opencv4 under android, by arranging some paths in the package folder.

• What does your PR fix? Fixes issue [opencv4:android] build failure (folder /include is empty or not present) #11229

• Which triplets are supported/not supported?
  Android triplets

• Have you updated the CI baseline?
  No

• Does your PR follow the maintainer guide?
  Yes

---

Additional details

Note: Beware, I am new to the android platform, and I did not succeed in trying out the resulting opencv4 package on an android application ("find_package(OpenCV)" fails).
May be @atkawa7 could help with some instructions on this: below I explain how I can succesfully export a prefab package. My issue is that I do not know how to consume it in an android app

Current status of opencv4 under android

Currently the compilation of opencv4 for android leads to
the following folder structure:

packages/opencv4_arm_android/
    debug/
    lib/
    sdk/
         native/
             jni/
                 include/
                     opencv2/
    share/
    README.android

vcpkg complains about a missing include folder, and about the presence
of unwanted README.android file.

Steps to reproduce the error

1. Install the android triplets:

You can copy-paste the script below to populate them (and maybe later adjust them to your needs):

echo "
set(VCPKG_TARGET_ARCHITECTURE arm)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic)
set(VCPKG_CMAKE_SYSTEM_NAME Android)
" > triplets/community/arm-android.cmake

echo "
set(VCPKG_TARGET_ARCHITECTURE arm64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic)
set(VCPKG_CMAKE_SYSTEM_NAME Android)
" > triplets/community/arm64-android.cmake

echo "
set(VCPKG_TARGET_ARCHITECTURE x86)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic)
set(VCPKG_CMAKE_SYSTEM_NAME Android)
" > triplets/community/x86-android.cmake

echo "
set(VCPKG_TARGET_ARCHITECTURE x64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic)
set(VCPKG_CMAKE_SYSTEM_NAME Android)
" > triplets/community/x64-android.cmake

2. Install android ndk

3. Set ANDROID_NDK_HOME env variable, for example:

export ANDROID_NDK_HOME=/home/YourAccount/Android/Sdk/ndk-bundle

4. Try to build opencv4[core]:

./vcpkg install "opencv4[core]:arm-android"

You will get the following error

-- Installing: .../vcpkg/packages/opencv4_arm-android/share/opencv4/copyright
-- Performing post-build validation
The folder /include is empty or not present. This indicates the library was not correctly installed.
The following files are placed in
.../vcpkg/packages/opencv4_arm-android:

    .../vcpkg/packages/opencv4_arm-android/README.android

Files cannot be present in those directories.

The following files are placed in
.../vcpkg/packages/opencv4_arm-android/debug:

    .../vcpkg/packages/opencv4_arm-android/debug/README.android

Files cannot be present in those directories.

Found 3 error(s). Please correct the portfile:
    ../vcpkg/ports/opencv4/portfile.cmake
-- Performing post-build validation done
Error: Building package opencv4:android-armeabi-v7a failed with: POST_BUILD_CHECKS_FAILED

What this patch does

It reorganizes the package folder post install, so that it contents looks like:

packages/opencv4_arm-android
├── debug
│   ├── lib
│   └── sdk
│       └── native
├── include
│   └── opencv2
│       ├── calib3d
│       ├── core
│       └── ...
├── lib
└── share
    ├── opencv
    │   ├── abi-armeabi-v7a
    │   └── licenses
    └── opencv4

How to test this patch

1. Build opencv for all 4 archs

./vcpkg install "opencv4[core]:arm-android" "opencv4[core]:arm64-android" "opencv4[core]:x64-android" "opencv4[core]:x86-android"

2. Export opencv4 prefab:

 ./vcpkg export --triplet x64-android "opencv4[core]" --prefab

You will get the following prefab structure

prefabo/opencv4
├── aar
│   ├── AndroidManifest.xml
│   ├── META-INF
│   │   └── LICENSE
│   └── prefab
│       ├── modules
│       │   ├── opencv_calib3d
│       │   │   ├── libs
│       │   │   │   ├── android.arm64-v8a
│       │   │   │   │   ├── abi.json
│       │   │   │   │   ├── include
│       │   │   │   │   │   └── opencv2
│       │   │   │   │   └── libopencv_calib3d.so
│       │   │   │   ├── android.armeabi-v7a
│       │   │   │   │   ├── abi.json
│       │   │   │   │   ├── include
│       │   │   │   │   │   └── opencv2
│       │   │   │   │   └── libopencv_calib3d.so
│       │   │   │   ├── android.x86
│       │   │   │   │   ├── abi.json
│       │   │   │   │   ├── include
│       │   │   │   │   │   └── opencv2
│       │   │   │   │   └── libopencv_calib3d.so
│       │   │   │   └── android.x86_64
│       │   │   │       ├── abi.json
│       │   │   │       ├── include
│       │   │   │       │   └── opencv2
│       │   │   │       └── libopencv_calib3d.so
│       │   │   └── module.json
│       │   ├── opencv_core
│       │   │   ├── libs
│       │   │   │   ├── android.arm64-v8a
│       │   │   │   │   ├── abi.json
│       │   │   │   │   ├── include
│       │   │   │   │   │   └── opencv2
│       │   │   │   │   └── libopencv_core.so

[cenit]

In general, renaming files after build is the source of many problems when consuming the library in a downstream project

Can you please try my pr with opencv 4.3 update, #11130? It might solve the include issue for android toolchain

[pthom]

In general, renaming files after build is the source of many problems when consuming the library in a downstream project

Can you please try my pr with opencv 4.3 update, #11130? It might solve the include issue for android toolchain

Many thanks for this informed answer!

Your PR seems to do quite a lot to solve the issue, expect for the remaining README.android files.
I will close my PR and add a comment to yours.