Add a method in bp2build to create aliases in another directory

The expected use case for this is to create aliases for stub libraries
in the @api_surfaces repository in build/bazel/api_surfaces.

This restricts the scope to just aliases. If we have a use case for
generating actual Bazel targets in another directory, a workaround could
be to generate the targets in the current directory (via
CreateBazelTargetModule) and aliases to it in the other directory

Test: go test ./bp2build
Change-Id: I6b63d9d018618d447fc7c260a2a94aaa00e57a4d