Build backend: Add fast path (#9556)

Going through PEP 517 to build a package is slow, so when building a
package with the uv build backend, we can call into the uv build backend
directly. This is the basis for the `uv build --list`.

This does not enable the fast path for general source dependencies.

There is a possible difference in execution if the latest uv version is
newer than the one currently running: The PEP 517 path would use the
latest version, while the fast path uses the current version.

Please review commit-by-commit

### Benchmark

`built_with_uv`, using the fast path:
```
$ hyperfine "~/projects/uv/target/profiling/uv build"
Time (mean ± σ):       9.2 ms ±   1.1 ms    [User: 4.6 ms, System: 4.6 ms]
Range (min … max):     6.4 ms …  12.7 ms    290 runs
```

`hatcling_editable`, with hatchling being optimized for fast startup
times:
```
$ hyperfine "~/projects/uv/target/profiling/uv build"
Time (mean ± σ):     270.5 ms ±  18.4 ms    [User: 230.8 ms, System: 44.5 ms]
Range (min … max):   250.7 ms … 298.4 ms    10 runs
```

docs/reference/cli.md

@@ -7888,6 +7888,10 @@ uv build [OPTIONS] [SRC]
 <p>Normally, configuration files are discovered in the current directory, parent directories, or user configuration directories.</p>
 
 <p>May also be set with the <code>UV_NO_CONFIG</code> environment variable.</p>
+</dd><dt><code>--no-fast-path</code></dt><dd><p>Always build through PEP 517, don&#8217;t use the fast path for the uv build backend.</p>
+
+<p>By default, uv won&#8217;t create a PEP 517 build environment for packages using the uv build backend, but use a fast path that calls into the build backend directly. This option forces always using PEP 517.</p>
+
 </dd><dt><code>--no-index</code></dt><dd><p>Ignore the registry index (e.g., PyPI), instead relying on direct URL dependencies and those provided via <code>--find-links</code></p>
 
 </dd><dt><code>--no-progress</code></dt><dd><p>Hide all progress outputs.</p>