Pass ProtoInfo instead of Target to proto_path (#1985)

During a refactor in 8670623e6e3a18123129173ba667c33631aca006, a
`.proto` was missed and now a Target gets passed into the `proto_path`
function rather than a `ProtoInfo` provider.

When used with `{strip_}import_prefix`, this would result in the
`import` flags passed to the `protoc-go` builder to contain incorrect
paths:
    tests/core/go_proto_library/_virtual_imports/adjusted_c_proto/adjusted/adjusted_c.proto=adjusted/c
instead of
    adjusted/adjusted_c.proto=adjusted/c

When `go_package` isn't present in a proto file, protoc-gen-go relies in
`Mfile.proto=go/importpath` to resolve proto files to import paths.
With these passed incorrect packages for dependencies fail to compile.
5 files changed