Warn when an import can't be resolved

[jayconrod]

(Originally bazelbuild/rules_go#388)

Gazelle should warn developers when an import can't be resolved into a Bazel dependency.

Now that we have a library index, we can tell definitively whether this is an error for local imports (under some known prefix) and for non-local imports in vendored mode (since we index vendor folders). We don't yet have a reliable way to resolve imports in external repositories.