Make sure repo.url points to a git repo before adding (#451)
* Check that repo urls point to a git repo
* Move `validate_repo_url` to `GitTools` module
* Add Pkg-specific error messages to GitTools `clone` and `fetch`
* Alter a test to be Windows-compatible
(cherry picked from commit 046bde4d660715291f6a84eaba3cd08917dc7cee)
