Add support for named and explicit indexes (#7481)

## Summary

This PR adds a first-class API for defining registry indexes, beyond our
existing `--index-url` and `--extra-index-url` setup.

Specifically, you now define indexes like so in a `uv.toml` or
`pyproject.toml` file:

```toml
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cu121"
```

You can also provide indexes via `--index` and `UV_INDEX`, and override
the default index with `--default-index` and `UV_DEFAULT_INDEX`.

### Index priority

Indexes are prioritized in the order in which they're defined, such that
the first-defined index has highest priority.

Indexes are also inherited from parent configuration (e.g., the
user-level `uv.toml`), but are placed after any indexes in the current
project, matching our semantics for other array-based configuration
values.

You can mix `--index` and `--default-index` with the legacy
`--index-url` and `--extra-index-url` settings; the latter two are
merely treated as unnamed `[[tool.uv.index]]` entries.

### Index pinning

If an index includes a name (which is optional), it can then be
referenced via `tool.uv.sources`:

```toml
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cu121"

[tool.uv.sources]
torch = { index = "pytorch" }
```

If an index is marked as `explicit = true`, it can _only_ be used via
such references, and will never be searched implicitly:

```toml
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cu121"
explicit = true

[tool.uv.sources]
torch = { index = "pytorch" }
```

Indexes defined outside of the current project (e.g., in the user-level
`uv.toml`) can _not_ be explicitly selected.

(As of now, we only support using a single index for a given
`tool.uv.sources` definition.)

### Default index

By default, we include PyPI as the default index. This remains true even
if the user defines a `[[tool.uv.index]]` -- PyPI is still used as a
fallback. You can mark an index as `default = true` to (1) disable the
use of PyPI, and (2) bump it to the bottom of the prioritized list, such
that it's used only if a package does not exist on a prior index:

```toml
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cu121"
default = true
```

### Name reuse

If a name is reused, the higher-priority index with that name is used,
while the lower-priority indexes are ignored entirely.

For example, given:

```toml
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cu121"

[[tool.uv.index]]
name = "pytorch"
url = "https://test.pypi.org/simple"
```

The `https://test.pypi.org/simple` index would be ignored entirely,
since it's lower-priority than `https://download.pytorch.org/whl/cu121`
but shares the same name.

Closes #171.

## Future work

- Users should be able to provide authentication for named indexes via
environment variables.
- `uv add` should automatically write `--index` entries to the
`pyproject.toml` file.
- Users should be able to provide multiple indexes for a given package,
stratified by platform:
```toml
[tool.uv.sources]
torch = [
  { index = "cpu", markers = "sys_platform == 'darwin'" },
  { index = "gpu", markers = "sys_platform != 'darwin'" },
]
```
- Users should be able to specify a proxy URL for a given index, to
avoid writing user-specific URLs to a lockfile:
```toml
[[tool.uv.index]]
name = "test"
url = "https://private.org/simple"
proxy = "http://<omitted>/pypi/simple"
```

crates/uv-client/src/registry_client.rs

@@ -1,6 +1,7 @@
 use async_http_range_reader::AsyncHttpRangeReader;
 use futures::{FutureExt, TryStreamExt};
 use http::HeaderMap;
+use itertools::Either;
 use reqwest::{Client, Response, StatusCode};
 use reqwest_middleware::ClientWithMiddleware;
 use std::collections::BTreeMap;
@@ -16,7 +17,7 @@ use uv_configuration::KeyringProviderType;
 use uv_configuration::{IndexStrategy, TrustedHost};
 use uv_distribution_filename::{DistFilename, SourceDistFilename, WheelFilename};
 use uv_distribution_types::{
-    BuiltDist, File, FileLocation, IndexCapabilities, IndexUrl, IndexUrls, Name,
+    BuiltDist, File, FileLocation, Index, IndexCapabilities, IndexUrl, IndexUrls, Name,
 };
 use uv_metadata::{read_metadata_async_seek, read_metadata_async_stream};
 use uv_normalize::PackageName;
@@ -204,8 +205,15 @@ impl RegistryClient {
     pub async fn simple(
         &self,
         package_name: &PackageName,
+        index: Option<&IndexUrl>,
     ) -> Result<Vec<(IndexUrl, OwnedArchive<SimpleMetadata>)>, Error> {
-        let mut it = self.index_urls.indexes().peekable();
+        let indexes = if let Some(index) = index {
+            Either::Left(std::iter::once(index))
+        } else {
+            Either::Right(self.index_urls.indexes().map(Index::url))
+        };
+
+        let mut it = indexes.peekable();
         if it.peek().is_none() {
             return Err(ErrorKind::NoIndex(package_name.to_string()).into());
         }