Add proto.local_include_dirs support in bp2build

This is a followup to aosp/2711093 which added support for
proto.include_dirs. local_include_dirs is simlar, except that is
relative to the module directory.

Test: go test ./bp2build
Bug: 285140726
Change-Id: I32ddc7371048672d6935f827c8aee9d767305e2c