Add some of 'buildtools/' subfolders to the '.bazelignore' file.

Some of the deps used for the GN build (installed via
'tools/install-build-deps') pulls Bazel build files that are
placed into `buildtools/` subfolders, e.g.: `buildtools/benchmark`.

If we later use `bazel query` to check the structure of our Blaze
build, these files are considered part of our Blaze project,
leading to obscure errors about missing `@@py_deps//`

We want to exclude all the dirs that contains a BUILD or
BUILD.bazel file, because Bazel considers such dirs as part of
the project, EXEPT for the 'buildtools/expat' dir, that is a part
of the build and should be kept.

The following command is used to generate the list:
`find buildtools -regex ".*/BUILD\(\.bazel\)?" | xargs -I {} sh -c 'echo "{}" | awk "{ print \$1, \$2 }" FS=/ OFS=/' | uniq | grep -v expat`

Tested:
bazel query //...
bazel build :all

Bug: N/A.
Change-Id: I6db32ec022c5fcc35afc5e63a5b3a24afc1c5b7f

Author: KirillTim

.bazelignore

@@ -1 +1,17 @@
-ui
+ui
+
+# Blaze 8 has an API to specify a glob of dirs to ignore:
+# https://bazel.build/versions/8.0.0/rules/lib/globals/repo#ignore_directories.
+# But we are using Blaze 7 and so explicitly lists the directories.
+#
+# We want to exclude all the dirs that contains a BUILD or BUILD.bazel file,
+# because Bazel considers such dirs as part of the project, EXEPT for the
+# 'buildtools/expat' dir, that is a part of the build and should be kept.
+# The following command is used to generate the list:
+# $ find buildtools -regex ".*/BUILD\(\.bazel\)?" | \
+# xargs -I {} sh -c 'echo "{}" | awk "{ print \$1, \$2 }" FS=/ OFS=/' | \
+# uniq | grep -v expat
+buildtools/googletest
+buildtools/protobuf
+buildtools/ndk
+buildtools/benchmark