`license_text` should not be mandatory

[Adam Azarchs]

Expected Behavior

It should be possible to create a license that does not set a license_text.

While it's certainly not a great idea to not have a license_text for a license, that's something that should be enforced (at the user's discretion) in compliance rules, rather than in the structure of the rule declarations. There are many situations where there isn't a practical alternative, e.g. downloading a python wheel where you know the SPDX ID of the license but the file isn't included in the zip file.

Actual Behavior

The macro wrapper sets license_text to LICENSE. This may be convenient in some cases, but at the same time that kind of invisible, implicit source file dependency is generally not the way most things work in bazel.