Rather temporary solution before proper:
https://github.com/neondatabase/neon/issues/5006
It requires more plumbing so lets not attach deleted tenants first and
then implement resume.
Additionally fix `assert_prefix_empty`. It had a buggy prefix calculation,
and since we always asserted for absence of stuff it worked. Here I
started to assert for presence of stuff too and it failed. Added more
"presence" asserts to other places to be confident that it works.
Resolves [#5016](https://github.com/neondatabase/neon/issues/5016)
Previously list_prefixes was incorrectly used for that purpose. Change
to use list_files. Add a test.
Some drive by refactorings on python side to move helpers out of
specific test file to be widely accessible
resolves https://github.com/neondatabase/neon/issues/4499
## Problem
neon_fixtures.py has grown to unmanageable size. It attracts conflicts.
When adding specific utils under for example `fixtures/pageserver`
things sometimes need to import stuff from `neon_fixtures.py` which
creates circular import. This is usually only needed for type
annotations, so `typing.TYPE_CHECKING` flag can mask the issue.
Nevertheless I believe that splitting neon_fixtures.py into smaller
parts is a better approach.
Currently the PR contains small things, but I plan to continue and move
NeonEnv to its own `fixtures.env` module. To keep the diff small I think
this PR can already be merged to cause less conflicts.
UPD: it looks like currently its not really possible to fully avoid
usage of `typing.TYPE_CHECKING`, because some components directly depend
on each other. I e Env -> Cli -> Env cycle. But its still worth it to
avoid it in as many places as possible. And decreasing neon_fixture's
size still makes sense.
