loader: Implement uniform loading CLI

• swh/devel/swh-loader-core!374 (closed): under the swh cli: swh loader <type> <url> <options>
• all loaders (including the registering step within setup.py's entry_points mechanism)
  • swh/devel/swh-loader-core!374 (closed): loader-package (archive, debian, deposit, npm, pypi)
  • swh/devel/swh-loader-git!129 (closed): git
  • D2399: mercurial
  • swh/devel/swh-loader-svn!193 (closed): svn

Note: This also implies some unification work for the different loader's instantiation and trigger method call (load here). The way forward, unifying with the lister, is to push the loader's state to be initialized within the constructor. And to have the main method (load) to have no parameter whatsoever. (This has been done in the multiple diffs referenced earlier).

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Migrated from T2134 (view on Phabricator)