Rename `distribution` to `packages` in lockfile (#5861)
Currently, the entry for a package+version+source table is called
`distribution`. That is incorrect, the `sdist` and `wheel` fields inside
of that table are distributions, the table itself is for a package. We
also align ourselves closer with PEP 751.
I went through `lock.rs` and renamed all occurrences of "distribution"
that actually referred to a "package".
This change invalidates all existing lockfiles.
Bikeshedding: Do we call it `package` or `packages`? See also
https://github.com/python/peps/pull/3877
`package` is nice because it looks like a header:
```toml
[[package]]
name = "anyio"
version = "4.3.0"
source = { registry = "https://pypi.org/simple" }
dependencies = [
{ name = "idna" },
{ name = "sniffio" },
]
sdist = { url = "https://files.pythonhosted.org/packages/db/4d/3970183622f0330d3c23d9b8a5f52e365e50381fd484d08e3285104333d3/anyio-4.3.0.tar.gz", hash = "sha256:f75253795a87df48568485fd18cdd2a3fa5c4f7c5be8e5e36637733fce06fed6", size = 159642 }
wheels = [
{ url = "https://files.pythonhosted.org/packages/14/fd/2f20c40b45e4fb4324834aea24bd4afdf1143390242c0b33774da0e2e34f/anyio-4.3.0-py3-none-any.whl", hash = "sha256:048e05d0f6caeed70d731f3db756d35dcc1f35747c8c403364a8332c630441b8", size = 85584 },
]
```
`packages` is nice because the field is not a single entry, but a list.
2/3 for https://github.com/astral-sh/uv/issues/4893
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
