Document/update --check_licenses and its interaction with distribs() and licenses()

[schroederc]

Source context: src/main/java/com/google/devtools/build/lib/packages/License.java

Since Bazel forces licenses to be set for //third_party/... targets, it would be very helpful to document what those declarations actual do and how --check_licenses works.

Likewise, I think that the current implementation is very Google-centric and could use some massaging to work better for the open-source community. In particular, the distribs() doesn't make a lot of sense for completely open-source repositories. It's often necessary to differentiate between executable and library distributions for some licenses, not just client vs web vs embedded.

Since the world of licenses is vast, it will also likely be necessary to provide some way to configure/extend this feature. In my opinion, this feature could mostly boil down to a tagging mechanism that infects/affects all dependent targets coupled with asserts on which tags can/must exist for a particular target.

[gregestren]

There's been talk of overhauling this mechanism even within Google - I think this is due for a design refresh, and your points just add fuel to the argument.

Depending on exactly what you want to do, you may also want to check out the (new and not yet documented) constraints system. The basic gist of it is that you can annotate any rule with tags of your choosing and enforce that if rule A supports "foo" and A depends on B, then B must also support "foo".

Feel free to ask for more details if you think that could help you.

[trainman419]

+1; this mechanism needs better documentation. This showed up again in #642 .

[michaelsafyan]

+1000. I think it is also reasonable in the opensource version to have --check_licenses default to false.