Allow to detect wellknown types in nested messages

[becoded]

Nested well-known types were not detected in nested messages:

message Foo {
  message Bar {
    string uuid = 1 [(validate.rules).string.uuid = true];
  }
}

would not add the required regex to the generated file as the UUID rule was not found.

Solving #462

[rodaine]

Great fix, thanks! Can we minimize the changes on this PR to those addressing the bug (so none of the Dockerfile/Makefile/go.mod diffs)? I'd be happy to approve those upgrades in a separate PR.

[becoded]

@rodaine just pushed the changes so it only reflects addressing the bug!

[rodaine]

Thanks!

[becoded]

Thanks for the quick follow up!
Any chance of getting a tag bump now this is merged @rodaine?