Fix replacements of namespace module srcs in genrule

expandSrcsForBazel always prefixed : in OriginalModuleName. The
exceptions to this are filegroups that appear in a different soong
namespace. For these cases, we were not correctly substituting the soong
module name with the equivalent bazel label.

Test: go test ./bp2build
Change-Id: If090f3f8819835177c1f4d191b3eef6bb6e30ace