Lockfiles: allow forcing a 3rd party requirement to be included in a resolve

[Eric-Arellano]

Motivating case: @benjyw pointed out how pantsbuild/pants depends on pydevd-pycharm. He'd like it to be in our user_reqs.txt lockfile, even though it's not imported by default so not actually used by the resolve. This allows you to quickly add the dependency when debugging without re-generating and resolving the lockfile.

We should allow you to ensure that a certain python_requirement_library gets included in the input for a user resolve no matter if it's used. Probably this should be a field on python_requirement? Otherwise, it could be an option in [python-setup] that associates resolves w/ certain 3rdparty requirements.

[Eric-Arellano]

This is solved via #13621 (comment). Regardless of if any code is using the requirement or not, if the python_requirement sets compatible_resolves, then it will be included in that lockfile. (And then we might subset that repository PEX to ignore it if the code doesn't use it, depending on --run-against-entire-lockfile)