`proto_strip_import_prefix` does not detect prefix correctly

Consider `foo/BUILD`:
```starlark
# gazelle:proto_strip_import_prefix /portal/protos
```

Gazelle prints the following error:
```
gazelle: proto_strip_import_prefix "/foo/bar" not in directory foo
```

Obviously `foo/bar` is in the directory `foo`, so this warning seems to be a false positive.