Revert "Allow overriding of undefined dependency SCM"

This reverts commit 5dd7a2ba3a08df89c5dd159bf66a4b22fe1a3f3f.

Conflicts:
	lib/mix/lib/mix/dep.ex
	lib/mix/lib/mix/dep/loader.ex
	lib/mix/test/fixtures/deps_status/custom/noscm_repo/mix.exs
	lib/mix/test/mix/tasks/deps_test.exs
Improve hex workflow

We do this by automatically starting hex, if one is available,
and by providing a local.hex task, similar to how we handle Rebar.

Note that this does not make Mix depend on Hex, we do this on a
best effort basis (similar to Rebar).