Local resolver should support multiple prefixes

[jayconrod]

Gazelle allows prefixes to be set in multiple directories using the # gazelle:prefix directive. This is used to determine importpath attributes for library rules. The resolver should use these when it can't resolve labels using the index.

For example, say we have the directive # gazelle:prefix example.com/foo set in the build file src/foo/BUILD.bazel. When we attempt to resolve the import path example.com/foo/bar, the local resolver should return //src/foo/bar:go_default_library. Currently, this would go to the external resolver.