…ference is unused (#10582)

### Problem

As of #10511, we now use file addresses / generated subtargets everywhere, except for the graph introspection rules.

By default, generated subtargets do not depend on the sibling files in their base target, i.e. the other files in the original `sources` field. Those dependencies will only be included if dependency inference added it, or if the user explicitly added something like `./sibling.txt` to the base target's `dependencies` field.

We generally do not want to automatically depend on sibling files. This would mean that file addresses (and file dependencies) do not actually have any benefit, because `./pants dependencies $file` would resolve the same results as `./pants dependencies $sibling`, i.e. we would not get finer-grained caching. Further, the conceptual model of "targets as metadata applied to files"—rather than "a collection of files with metadata"—would imply that we should not depend on sibling files automatically.

However, for users who are not using dep inference and/or explicit file deps, many things will break if we don't automatically depend on sibling files. In Pants 1.x, it was a safe assumption that you could access any file in the target's `sources` field without having to add a dependency. It's majorly disruptive to break this assumption, and in effect, they would need to use dependency inference to be ergonomic.

### Solution

Unless `--python-infer-imports` is True, and the target in question has `PythonSources`, then we will automatically add dependencies on all sibling files for file addresses. 

This means that every non-Python target (e.g. `files`, `bash_library`) will depend on its siblings, regardless of using dependency inference.

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…10619)

In #10603, we moved `conftest.py` from `python_tests` to `python_library`. This is because Pants now runs file-at-a-time for Pytest thanks to the model change in #10511. This causes `conftest.py` to error because it doesn't have any test files.

We decided in #10603 that `conftest.py` should be under `python_library` because it is not actual tests. For example, test util code goes under a `python_library` target already.

However, `python_library` owning a `conftest.py` by default ended up being problematic, as it caused test code to be mixed with production code, which is generally not desired. See #10613 for an approach to fix this.

Instead of adding a `conftest` target, this PR instead moves back `conftest.py` to `python_tests` and special cases `pytest_runner.py` to skip `conftest.py`. Even though this is less correct, it's simpler for users and it avoids making 1.x users having to change a bunch of things. Conceptually, `conftest.py` can be seen as "config" for tests, rather than traditional "test utils" like you'd have in a `python_library`.

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…`fmt` (#10630)

Thanks to #10511, targets are no longer an atomic unit Pants runs on. Instead, files are the atomic unit.

This option rename reflects that model change.

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