Extract transport_find_proper_fn().

This function just returns proper transport, i.e., non-local ones.
Introduced git_transport_supported_proper_scheme().

This function can be used to query if a given URL contains a proper scheme
and if this scheme is supported by libgit2. Local file paths are not
considered to bear a proper scheme.
Don't check for the path if a proper URL has been specified.

A proper URL is not considered as local hence it doesn't make sense to
check whether the URL is actually existent on the local filesystem.