Add tooling & documentation for dealing with file collisions

[adisbladis]

Problem

It's a relatively common packaging bug to write files such as README.txt & LICENSE outside of the intended directories:

       > FileCollisionError: Two or more packages are trying to provide the same file with different contents
       >
       >         Files: /nix/store/76fd84qmzg7n9w5yif61cmv5rr80i686-alive-progress-3.2.0/LICENSE /nix/store/r4hn0hx1bjdrshik3cw9f6h33vwk7iii-about-time-4.2.1/LICENSE

You can already work around around this issue by overriding a package & deleting the colliding files from the origin package:

pyprojectOverrides = final: prev: {
  alive-progress = prev.alive-progress.overrideAttrs(_: { postInstall = "rm $out/LICENSE"; });
  about-time = prev.about-time.overrideAttrs(_: { postInstall = "rm $out/LICENSE"; });
}

But this is less than ideal and should be able to be fixed in the venv creation script instead.

Solutions

I see three possible, not mutually exclusive, solutions to the problem in the venv creation:

• Ignore collisions for a certain path:

(mkVirtualEnv "foo-env" { foo = []; }).overrideAttrs(old: {
  venvIgnoreCollisions = [
    "lib/python${final.python.pythonVersion}/site-packages/build_tools"
  ];
})

• Skip a path completely:

(mkVirtualEnv "foo-env" { foo = []; }).overrideAttrs(old: {
  venvSkip = [
    "lib/python${final.python.pythonVersion}/site-packages/build_tools"
  ];
})

• Control which path is the origin for a certain path:

(mkVirtualEnv "foo-env" { foo = []; }).overrideAttrs(old: {
  venvOverrides = {
    "lib/python${final.python.pythonVersion}/site-packages/build_tools" = "${pythonSet.foo}/lib/python${final.python.pythonVersion}/site-packages/build_tools";
  };
})