Stop trasforming "scp-style" URIs into normal URIs in Git

Summary:
See D3033, T1529. We currently transform "scp-style" user@host:path URIs into normal ssh://user@host/path URIs. This is undesirable for two reasons:

• The paths aren't always equivalent. They are for GitHub, which is why I missed this originally, but in the general case the ":path" is resolved relatively and the "/path" is resolved absolutely. So this transformation can break things.
• It confuses users, who do not think of "git@host:path" URIs as SSH URIs even though the SSH protocol is implied.

So stop using them, and just use the "git@host:path" URIs instead. This is a bit messy since we have some validation built up on top of URIs. Hopefully we can get rid of more of this in the future as we simplify repository management.

Test Plan: Unit tests cover this stuff pretty well. Made a new git repository with a "git@host:path" style URI and did pull/discover on it, verified the right URI was used.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1529

Differential Revision: https://secure.phabricator.com/D3036