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.
Test Plan
tox (happy)
Migrated from D6310 (view on Phabricator)