Allow rulesets to adopt rules_license

[fmeum]

While trying out rules_license for a ruleset I maintain, I noticed three problems that made it difficult for me to use:

1. The well-known license_kinds are not publicly visible (fixed by Make default license_kind definitions public #7).
2. It is not possible to set the visibility of license targets (fixed by Forward kwargs to _license #8).
3. There is no release, so the repository can't be safely loaded via http_archive. This turned out to be an "urban legend", see Should rulesets distribute a pre-built artifact rather than rely on GitHub source/release archive bazel-contrib/SIG-rules-authors#11 (comment).

Is rules_license considered to be in a stable enough state that rulesets could already use it to add applicable_licenses with well-known license_kinds to their public targets? If so, it would be great if these three problems could be resolved.

[fmeum]

@aiuto Friendly ping.

[aiuto]

I am starting to work on this now. Stay tuned. And maybe read https://docs.google.com/document/d/1XszGbpMYNHk_FGRxKJ9IXW10KxMPdQpF5wWbZFpA4C8/edit#

[aiuto]

Believed fixed. We are using this from rules_pkg, and within Bazel itself.
It's still a very slim feature set. I'm using Bazel as a test case to wind through a lot of issues related to adding license attestation to rules that don't have it. That is higher priority for me than adding features, because the features don't do much good if you have no license attestations to drive them.