MSYS2 dependencies

[emptyVoid]

Describe the pull request

Adds tool-msys metaport to update MSYS2 installation and rebuild the ports using vcpkg_acquire_msys when the latter is changed (it's a maintainer's responsibility to increment the metaport's version when making changes to vcpkg_acquire_msys).

Sets <vcpkg>/downloads directory as an MSYS2 package cache to reuse them after MSYS2 updates.

Additionally introduces PORT_VERSION variable providing access to the version string from port's CONTROL file to CMake scripts.

• What does your PR fix?
  Fixes [icu] build failure #11873

• Which triplets are supported/not supported? Have you updated the CI baseline?
  Not applicable.

• Does your PR follow the maintainer guide?
  Yes.

Waiting on #11836 to conclude, as it introduces a dependency metaport for vcpkg_configure_make which itself is dependent on vcpkg_acquire_msys.

[cenit]

isn't it better here and in #11836 to just rework the update mechanism and use not just CONTROL version, but also hashes like in CI to verify if update is necessary?
In this way we can avoid this meta port thing, which might "explode" if applied to all tools and toolchains...

[emptyVoid]

Well, of course it would be better if the update mechanism checked changes in dependency CMake scripts in addition to CONTROL file. To build the dependency list it would most likely need the portfile's abstract syntax tree which afaik is not publicly accessible, although it should be possible to leverage CMake's sources and cmListFile::ParseFile method directly. But that's a whole 'nother story.

[Neumann-A]

This PR is no longer required since vcpkg now has a heuristic which detects script changes.

[JackBoosY]

I think we don't need this PR now.
If you have any concern, please let me know.

[emptyVoid]

AFAICT #12254 doesn't take into account transitive dependencies, such as portfile>vcpkg_configure_make>vcpkg_acquire_msys.