opam: Move the state initialization into the get_pages method

We should avoid side-effects in the constructor as much as possible. That avoids
surprising behavior at object instantiation time. The state if needed must be
initialized into the swh.lister.pattern.Lister.get_pages method, as preconized in the
class docstring.

This also fixes the current test that actually bootstrap a real opam local "clone" in
/tmp.

Related to T3590