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Change `conftest.py` to default to `python_library` rather than `python_tests` (#10603)

### Problem

`conftest.py` is (typically) a util file with no tests in it. Now that we always run tests one-file-at-a-time thanks to generated subtargets, this causes Pytest to try to run on `conftest.py`, which fails because there are no tests.

More philosophically, test util code has always meant to go into `python_library`, rather than `python_tests`. A target is "metadata for some files" - it doesn't make sense to say something like `timeout=120` on `conftest.py`.

### Solution

Change default source globs so that `conftest.py` will show up in `python_library()` now.

### Result

Pytest no longer will error if you have a `conftest.py` and were using default source globs.

If you already had a `python_library()` target in the directory where the `conftest.py` was defined—and you have `--python-infer-conftests` enabled (the default)—then you will not need to make any changes for this to work.

If you only had a `python_tests()` target—and you have `--python-infer-conftests`—you simply need to add `python_library(name='conftest')` to your BUILD file.

If you only had a `python_tests` target—and do not have `--python-infer-conftests`—you will need to add `python_library(name='conftest')` and also update your `python_tests()` to explicitly depend on that new target.

For new users, their BUILD file will now look something like this in a folder that solely has tests + `conftest.py`:

```
python_tests()

python_library(name="conftest")
```

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