Make py_wheel provide depset of files

The basic goal is to allow depending on a build-time generated wheel (i.e. `py_wheel` _without_ having to go through the zip/unzip process of creating a wheel archive. Wheels may be large in size, so the process of staging, archiving, and extracting them is unnecessary overhead.

So the idea is `py_wheel` provides `(prefix, files)` information, which can then be used by consumers. This then allows e.g.

```
py_wheel(name="foo_whl", ...)
py_wheel_as_py_library(
name = "foo_whl_as_pylib",
srcs = [":foo_whl"]
)
py_test(
name="foo",
deps = ["foo_whl_as_pylib"],
)
```

Where `py_wheel_as_py_library` returns PyInfo() et al pointing to what the whl would extract to (i.e. a directory with the contents of the wheel).

Looking at py_wheel, this looks quite doable, since Starlark is already generating most (if not all?) of what goes into the wheel, and the `wheelmaker.py` program is mostly just zip's the files up.

首先阅读现有的 py_wheel 实现和 wheelmaker.py，以了解 wheel 的内容及其前缀是如何生成的。定义 py_wheel 如何公开前缀和文件，以及 py_wheel_as_py_library 如何使用它们来提供 PyInfo()，而无需对归档进行 staging 或解压。完成的标准是：通过提议的依赖模式，py_test 可以使用构建时生成的 wheel。

由索引模型根据 Issue 内容生成。