fix: consistent clobbering & removal of __pycache__

[wolfv]

This attempts to fix two issues:

• remove empty __pycache__ folders for noarch packages (otherwise the package is still "detected" in the site-packages folder).
• do not attempt to link individual files more than once per installation. This happens when there are "clobbering" files. Currently a random file will win (due to async execution).

However, there is still two remaining issues with clobbering:

• We should deterministically decide on a "winner". I think it should be by topologically sorting the packages and take the last (top) package as a winner.
• When removing packages we also need to take care of the (un-)clobbering. This is a bit more tricky because we might not even have the clobbering package in the package cache anymore. However, I think a workaround could be to still link the files into the environment but with the name of the source package appended (e.g. crates2.json would become crates2.json-clobber-from-mypackage. For unclobbering we could just rename the file.

[wolfv]

cc @Wackyator we could also re-use the unlink_package in the Python bindings! :)

[baszalmstra]

Nice! I like the simplicity!

[Wackyator]

looks awesome, will add this to the bindings :)

[baszalmstra]

I like this a lot @wolfv ! Lets get it merged asap!

[wolfv]

OK, now I just need to write some tests for the unlinking parts and then I think we're done with this one :) ... the easiest would be to resolve some actual packages, install, then unlink them. But I don't think we have a test harness for that yet.

[wolfv]

@baszalmstra when you get around to review this, please make sure to really carefully review the unlink parts because if that goes south, we could delete user data which we definitely would not want!

[wolfv]

@baszalmstra I took care of your comments. Two things we might still want to do:

• test "deeper nested" clobbers (bla/bli/clobber.txt).
• add something about clobbers to conda-meta files.