Rename `LevelAndSource`

It is misnamed, given that it has three fields: `level`, `lint_id`, and
`src`. (Presumably the `lint_id` was added later.)

This commit renames it as `LevelSpec`. (I also considered `LevelInfo`
but that's very generic. `Spec` has pre-existing uses in
`LintLevelsProvider::current_specs` and `LintSet::specs` and
`ShallowLintLevelMap::specs`.)

Related things renamed as well:
- `level` -> `level_spec` (where appropriate)
- `lint_levels` -> `lint_level_specs`
- `get_lint_level` -> `get_lint_level_spec`
- `level_and_source` -> `level_spec`
- `CodegenLintLevels` -> `CodegenLintLevelSpecs`
- `raw_lint_id_level` -> `raw_lint_level_spec`
- `lint_level_at_node` -> `lint_level_spec_at_node`
- `reveal_actual_level` -> `reveal_actual_level_spec`
- `probe_for_lint_level` -> `probe_for_lint_level_spec`

This clears up a lot of `Level` vs. `LevelSpec` ambiguity. E.g. no more
`level.level` expressions.