This installs a `swh loader` subcommand.
This allows loading of url for a given loader.
This also aligns the npm loader's constructor to be consistent with other
loaders (having only one url as constructor parameter).
Related to T2134
Note:
It's not tested yet. Just to ensure this is going the right way first.
Also, this cannot pass the ci as it's built against my latest refactoring on
the loader which is not completely in diff (see D2395).