Build backend: Add direct builds to the resolver and installer (#9621)

This is like #9556, but at the level of all other builds, including the
resolver and installer. 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 instead: No temporary virtual env, no
temp venv sync, no python subprocess calls, no uv subprocess calls.

This fast path is gated through preview. Since the uv wheel is not
available at test time, I've manually confirmed the feature by comparing
`uv venv && cargo run pip install . -v --preview --reinstall .` and `uv
venv && cargo run pip install . -v --reinstall .`. When hacking the
preview so that the python uv build backend works without the setting
the direct build also (wheel built with `maturin build --profile
profiling`), we can see the perfomance difference:

```
$ hyperfine --prepare "uv venv" --warmup 3 \
    "UV_PREVIEW=1 target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --preview" \
    "target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --find-links target/wheels/"
Benchmark 1: UV_PREVIEW=1 target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --preview
  Time (mean ± σ):      33.1 ms ±   2.5 ms    [User: 25.7 ms, System: 13.0 ms]
  Range (min … max):    29.8 ms …  47.3 ms    73 runs
 
Benchmark 2: target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --find-links target/wheels/
  Time (mean ± σ):     115.1 ms ±   4.3 ms    [User: 54.0 ms, System: 27.0 ms]
  Range (min … max):   109.2 ms … 123.8 ms    25 runs
 
Summary
  UV_PREVIEW=1 target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --preview ran
    3.48 ± 0.29 times faster than target/profiling/uv pip install --no-deps --reinstall scripts/packages/built-by-uv --find-links target/wheels/
```

Do we need a global option to disable the fast path? There is one for
`uv build` because `--force-pep517` moves `uv build` much closer to a
`pip install` from source that a user of a library would experience (See
discussion at #9610), but uv overall doesn't really make guarantees
around the build env of dependencies, so I consider the direct build a
valid option.

Best reviewed commit-by-commit, only the last commit is the actual
implementation, while the preview mode introduction is just a
refactoring touching too many files.

crates/uv-build-backend/src/lib.rs

@@ -2,7 +2,7 @@ mod metadata;
 mod source_dist;
 mod wheel;
 
-pub use metadata::PyProjectToml;
+pub use metadata::{check_direct_build, PyProjectToml};
 pub use source_dist::{build_source_dist, list_source_dist};
 pub use wheel::{build_editable, build_wheel, list_wheel, metadata};