Use os.Stderr.Write instead of fmt.Fprint. The latter formats the
value as a []byte, not a string.

(This was due to my own bad review comment on #2736. Sorry.)

Use os.Stderr.Write instead of fmt.Fprint. The latter formats the
value as a []byte, not a string.

(This was due to my own bad review comment on #2736. Sorry.)

These are just bugfixes to already merged code:
* Fix nested bracket parsing in PEP508 marker parser.
* Fix the sys_platform constants, which I noticed in bazel-contrib#2629 but they got
  also pointed out in bazel-contrib#2766.
* Port some of python tests for requirement parsing and improve the
  implementation. Those tests will be removed in bazel-contrib#2629.
* Move the platform related code to a separate file.
* Rename `pep508_req.bzl` to `pep508_requirement.bzl` to follow the
  convention.


All of the bug fixes have added tests.

Work towards bazel-contrib#2423.

This change addresses a bug where `pip.parse` selects the wrong
requirement entry when multiple extras are listed with platform-specific
markers.

#### 🔍 Problem:
In a `requirements.txt` generated by tools like `uv` or `poetry`, it's
valid to have multiple entries for the same package, each with different
extras and `sys_platform` markers, for example:

```ini
optimum[onnxruntime]==1.17.1 ; sys_platform == 'darwin'
optimum[onnxruntime-gpu]==1.17.1 ; sys_platform == 'linux'
```

The current implementation in
[`[parse_requirements.bzl](https://github.com/bazel-contrib/rules_python/blob/032f6aa738a673b13b605dabf55465c6fc1a56eb/python/private/pypi/parse_requirements.bzl#L114-L126)`](https://github.com/bazel-contrib/rules_python/blob/032f6aa738a673b13b605dabf55465c6fc1a56eb/python/private/pypi/parse_requirements.bzl#L114-L126)
uses a sort-by-length heuristic to select the “best” requirement when
there are multiple entries with the same base name. This works well in
legacy `requirements.txt` files where:
```
my_dep
my_dep[foo]
my_dep[foo,bar]
```
...would indicate an intent to select the **most specific subset of
extras** (i.e. the longest name).

However, this heuristic **breaks** in the presence of **platform
markers**, where extras are **not subsets**, but distinct variants. In
the example above, Bazel mistakenly selects `optimum[onnxruntime-gpu]`
on macOS because it's a longer match, even though it is guarded by a
Linux-only marker.

#### ✅ Fix:
This PR modifies the behavior to:
1. **Add the requirement marker** as part of the sorting key.
2. **Then apply the longest-match logic** to drop duplicate requirements
with different extras but the same markers.

This ensures that only applicable requirements are considered during
resolution, preserving correctness in multi-platform environments.

#### 🧪 Before:
On macOS, the following entry is incorrectly selected:
```
optimum[onnxruntime-gpu]==1.17.1 ; sys_platform == 'linux'
```

#### ✅ After:
Correct entry is selected:
```
optimum[onnxruntime]==1.17.1 ; sys_platform == 'darwin'
```

close bazel-contrib/rules_python#2690

---------

Co-authored-by: Ignas Anikevicius <240938+aignas@users.noreply.github.com>