Normalize submodule urls before looking at them

Extract the backslash-to-slash conversion into a helper function.

Our path functions expect to work with slashes, so convert a
path with backslashes into one with slashes at the top of
the function.

If we get the path from the gitmodules file, look up the submodule we're
interested in by path, rather then by name. Otherwise we might get
duplicate results.

The regex we use to look at the gitmodules file does not correctly
delimit the name of submodule which we want to look up and puts '.*'
straight after the name, maching on any submodule which has the seeked
submodule as a prefix of its name.

Add the missing '\.' in the regex so we want a full stop to exist both
before and after the submodule name.

Remove some warnings

We allow looking up a submodule by path, but we lost the path
normalisation during the recent changes. Bring it back.

Remove some of the logic that was left-over from the time we had a cache
of submodules, plugging a leak of the submodule object in certain cases.

Fallback describes the mechanism, while unspecified explains what the
user is thinking.