This aligns the implementation closer to the implementation in docker/cli,
with the refactor done in [cli@260f1db]; this removes some direct uses of
the github.com/docker/docker/builder/remotecontext/urlutil package, which
won't be included in the new Moby modules.

There's still some remaining uses in the `dockerFilePath` utility (which
may need to be updated to also account for remote contexts that are not
"git"), so possibly we can remove the use in that utility as well.

[cli@260f1db]: docker/cli@260f1db

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>