Fix package boundary in glob expansion with checked in BUILD files.

For directories without an Android.bp file, if they contain a merged
checked in BUILD file, it becomes a package boundary in the symlink
forest. However, the current glob expansion is only aware of Android.bp
files, but not merged BUILD files, so it generates glob expansions
incorrectly for files that cross the package boundary.

This CL fixes that by making the package boundary function aware of the
keepExistingBuildFile allowlist. See the new test cases for example.

Also stop using the same global bp2buildConfig for every test case to
allow each test to define their own bp2build allowlists.

Bug: 185358476
Test: new unit tests

Change-Id: Ifcd1283480a09a642ef4343e5bbc599583524577