This fits with the style for the rest of the project, but more
importantly, makes life easier for bindings authors who auto-generate
code.

Reload the HEAD and index data for a submodule after reading the
configuration.  The configuration may specify a `path`, so we must
update HEAD and index data with that path in mind.

When searching for information about a submdoule, let's be more explicit
in what we expect to find. We currently insert a submodule into the map
and change certain parameters when the config callback gets called.

Switch to asking for the configuration we're interested in, rather than
taking it in an arbitrary order.

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.