Cannot declare a license on scala_import

Hello,

I tried the new scala_import rule, and stumbled upon the following error:

```
ERROR: [...]/third_party/java/com/twitter/BUILD:237:1: //third_party/java/com/twitter:finagle_http: no such attribute 'licenses' in 'scala_import' rule
ERROR: [...]/third_party/java/com/twitter/BUILD:237:1: third-party rule '//third_party/java/com/twitter:finagle_http' lacks a license declaration with one of the following types: notice, reciprocal, permissive, restricted, unencumbered, by_exception_only
```

It seems that is not possible either on other Scala rules (unless I am mistaken), and it is indeed required to load third-party Scala rules.

Reproduce the failure from third_party/java/com/twitter/BUILD, focusing on the scala_import target and its license declaration. Inspect the scala_import rule entry point and compare other Scala rules; done means a third-party scala_import target accepts a supported license and passes Bazel's license validation.

Written by the indexing model from the issue text.