Make PackageVarContext implement PathGlobContext

Make PackageVarContext implement PathGlobContext by implementing
GlobWithDeps.  This will allow calls to ExistentPathForSource
inside a VariableFunc to use optimized glob dependencies instead of
falling back to AddNinjaFileDeps, which is resulting in extra
dependencies from soong_build on top level directories, triggering
extra Soong regenerations.

Remove the fallback path in ExistentPathForSource by making it take
a PathGlobContext, which is now a superset of PathContext.

Rewrite TestNinjaDeps to not rely on the unoptimized glob dependencies
in VariableFuncs and instead call ctx.Config().AddNinjaFileDeps
directly.

Bug: 257079828
Test: test_create_global_include_directory
Change-Id: I48cf189157d78b9252d339dbc9baeb27e4694807