checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
We do not use a "plugin system" for Git addons anymore, and therefore this
directory is no longer useful. Indeed that directory is way more confusing,
given that it includes more than mere Git addons, going from Bitbucket server
command-line tools to complete rewrites of Git in exotic programming languages.
Also, without this directory, the mental load of decision-making reduces a lot.
When anyone is interested in including a new git-related tool, just put it into
pkgs/applications/version-management, without apologies.
