Specifying urls for [source.allow] should warn when not a base

[Jake-Shadle]

While we check that a url is actually a valid url, we should also warn if the url is incorrect for the purposes of matching, as we only need the base url, without any query or fragments, and without any leading schemes, eg git+

[Jake-Shadle]

Alternatively, we can just auto correct the url internally, but still give a warning, as it makes the config file less pleasant, and would give the user the false idea that they need to eg fully specify a revision for every source.