refactor: remove else clauses using Object Calisthenics #2

[odanilosalve]

Summary

Follow-up to #2816 (clean imports + next(iter())). Applies Object Calisthenics rule #2 (no else keyword) to maigret/checking.py, with the readability fixes the reviewer asked for.

Patterns applied

• Dict mapping: HTTP method selection collapses to a single getattr (no more per-call identity dict rebuilt on every request):

  method = method.lower()
  request_method = getattr(
      session, method if method in ('get', 'post', 'head') else 'get'
  )

• Guard clauses: make_site_result returns early for disabled sites, mismatched identifier types, and disallowed usernames — no more if/elif/else ladder.
• Early returns: SSL exception handling, payload form/json branching, request_method defaulting, and the allow_redirects ternary.
• Inline assignment: keyword_match_status is set in the same if arm that decides it (no separate line with a long ternary).
• Dict dispatch: process_site_result uses a _checkers dict instead of an if/elif/elif/elif/else chain. Each helper uses early returns — none of the long one-line ternaries that read worse than the original if/else.

Readability fixes from review

• _check_message / _check_status_code / _check_response_url use guard-then-return, not one-line ternaries.
• keyword_match_status is set inside the if/else arms instead of via a long ternary.
• is_form ternary is short and reads fine; left as-is.

Net change

-12 net lines in checking.py, 314 tests pass, 3 skipped (network-dependent).

Depends on #2816 for the import reorganization + next(iter()) cleanup.