pkg2: make the Reqs submodule entirely path-agnostic.
pkg2: Write.update_file(f, file, args...) filters and replaces a file.

This can be used, e.g., to add or remove requirements to REQUIRE:

	Write.update_file(Reqs.add, "REQUIRE", pkg, versions)
	Write.update_file(Reqs.rm,  "REQUIRE", pkg)