Discover packages through `site.getsitepackages()` entries
Previously, uv was discovering packages only in purelib/platlib, which excluded system site packages if activated.
`site.getsitepackages()` is more stable than `sys.path`. `sys.path` can change in a number of ways such as `.pth` files and runtime modification, while `site.getsitepackages()` is generally stable for a Python environment. Using the `site`, we get the same paths that Python itself uses for `sys.path`.
