Reorder vcs imports, to try more popular VCS systems sooner

[pradyunsg]

Closes #7162, but using the check-order optimisation suggested there rather than via adding additional checks.

I don't think this is worth calling out in the changelog though.

---

This changes the order of vcs_backend in

pip/src/pip/_internal/vcs/versioncontrol.py

Line 237 in 72e38ca

for vcs_backend in self._registry.values():

$ # before
$ pip freeze
<pip._internal.vcs.bazaar.Bazaar object at 0x7f1a34c8cac0>
<pip._internal.vcs.git.Git object at 0x7f1a34c950a0>
<pip._internal.vcs.mercurial.Mercurial object at 0x7f1a34c954c0>
<pip._internal.vcs.subversion.Subversion object at 0x7f1a34c959a0>
$ # after
$ pip freeze
<pip._internal.vcs.git.Git object at 0x7f04ceceefa0>
<pip._internal.vcs.subversion.Subversion object at 0x7f04cecf4400>
<pip._internal.vcs.mercurial.Mercurial object at 0x7f04cecf48b0>
<pip._internal.vcs.bazaar.Bazaar object at 0x7f04cecf4c40>

[uranusjr]

What scenario does this improve and how much?

[pradyunsg]

The scenario is elaborated upon in #7162. This should be ~130ms faster, based on that description, for that specific scenario.

That said, I'm not sure this is particularly valuable (which is why this has sat for so long with no updates). :)

[pradyunsg]

Closing this out. If someone else wants to pick this up and benchmark it, please feel welcome to do so.