daemon invalidation for source code modifications

[kwlzn]

after #5045 lands, it occurred to me that a simple strategy for invalidating the daemon on source code modification would be to fingerprint any relevant sources and then set the --version option implicitly in the pants wrapper with e.g. a suffixed sha. this should inherently provide for the appropriate restarts on source code modification while in development mode in the repo since the value of --version will be considered as part of the daemon's own fingerprint/lifecycle.

[stuhood]

cc @jsirois ... likely related to dogfooding of test caching as well.

[stuhood]

@kwlzn : I've updated the title. I think that (although it's potentially expensive), a general solution here would be to fingerprint everything in the PYTHONPATH (which pants itself adds to in order to load loose-source plugins).

This would have the effect of fixing the issue for loose source plugins as well: see https://www.pantsbuild.org/howto_plugin.html#simple-configuration

[stuhood]

I'm going to target this out of the beta, as it doesn't affect users.

[stuhood]

Mentioned in slack: this also affects loose source plugins in someone's repo.

[stuhood]

Resolved. There is a followup in #5567 though.