(feat request) Add a gazelle directive for using `requirement()` for pip-installable dependencies

# 🚀 feature request

### Relevant Rules

+ gazelle
+ requirements.bzl

### Description

Gazelle generates deps using the library labels directly, rather than using `requirement()` (possibly because of the [drawbacks of `requirement()`](https://github.com/bazelbuild/rules_python/issues/414)).

```starlark
# using labels
py_library(
name = "foo",
srcs = ["foo.py"],
deps = ["@pip_parse_name//boto3"],
)
```

```starlark
# using `requirement()` function
load("@pip_parse_name//:requirements.bzl", "requirement")

py_library(
name = "foo",
srcs = ["foo.py"],
deps = [requirement("boto3")],
)
```

It would be great if we could configure this on a per-package basis. Users would then be able to opt-in to using `requirement()`, making the transition from manually-built BUILD files to gazelle-generated files much easier (as the diff will be much smaller or nonexistent[^1]).

[^1]: The [docs](https://rules-python.readthedocs.io/en/latest/pypi-dependencies.html#using-third-party-packages) guide users to use `requirement()` initially, and I'm not sure how many people opt to use labels instead.

### Describe the solution you'd like

Add a new directive `gazelle:python_use_pip_requirement_function` (or similar name) that accepts 3 values:

+ **`false` (the default):** Use library labels directly. Same as current behavior.
+ **`true`:** Use `requirement("library")`
+ **`DEFAULT` or `RESET`:** reset to the default as defined by `rules_python`.

Example:

```starlark
# no directive set. Use labels directly.
py_library(
...,
deps = ["@pip_parse_name//boto3"],
)

# gazelle:python_use_pip_requirement_function false
py_library(
...,
deps = ["@pip_parse_name//boto3"],
)

# gazelle:python_use_pip_requirement_function true
load("@pip_parse_name//:requirements.bzl", "requirement")
py_library(
...,
deps = [requirement("boto3")],
)

# gazelle:python_use_pip_requirement_function DEFAULT
py_library(
...,
deps = ["@pip_parse_name//boto3"],
)
```

### Describe alternatives you've considered

None 🙃. I'm happy to hear some though.

Start by locating Gazelle's Python dependency generation and how requirements.bzl is integrated. Review the existing directive handling and dependency-label generation before deciding where the per-package setting belongs. Done means the directive supports true, false, and the requested reset behavior, with coverage for generated labels, requirement() calls, and the load statement.

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