Implement pylint import-outside-toplevel rule (C0415) (#5180)
## Summary
Implements pylint C0415 (import-outside-toplevel) — imports should be at
the top level of a file.
The great debate I had on this implementation is whether "top-level" is
one word or two (`toplevel` or `top_level`). I opted for 2 because that
seemed to be how it is used in the codebase but the rule string itself
uses one-word "toplevel." 🤷 I'd be happy to change it as desired.
I suppose this could be auto-fixed by moving the import to the
top-level, but it seems likely that the author's intent was to actually
import this dynamically, so I view the main point of this rule is to
force some sort of explanation, and auto-fixing might be annoying.
For reference, this is what "pylint" reports:
```
> pylint crates/ruff/resources/test/fixtures/pylint/import_outside_top_level.py
************* Module import_outside_top_level
...
crates/ruff/resources/test/fixtures/pylint/import_outside_top_level.py:4:4: C0415: Import outside toplevel (string) (import-outside-toplevel)
```
ruff would now report:
```
import_outside_top_level.py:4:5: PLC0415 `import` should be used only at the top level of a file
|
3 | def import_outside_top_level():
4 | import string # [import-outside-toplevel]
| ^^^^^^^^^^^^^ PLC0415
|
```
Contributes to https://github.com/astral-sh/ruff/issues/970.
## Test Plan
Snapshot test.
