uv: move all uv commands to uv.rs

[dsp]

The pull requests moves all invocations of uv to src/uv.rs. The invokations are now structured into three main structures:

• UvBuilder, which can be used to create a new instance of a uv binary that ensures uv exists on disk.
• Uv, which provides operations using uv outside of the virtual environment, for example creating a venv, compiling lockfiles ,etc.
• UvWithVenv, which must be obtained using Uv::venv() to guarantee a venv is available, which encapsulates operations on the venv.

This opens the door to implement the same patterns for pip-tools and intorduce a trait to use dynamic dispatching instead of the existing if/else structure. If we continue down this path, we can hopefully in the future just do:

// Create either PipTools or Uv instance, returns `Box<PipLikeTool>`
PipBuilder::use_uv(Config::use_uv())
   .build()
   .with_output(output)
   .venv(...);

Important Note:

Note that the changes are not 100% equivalent to the previous code. Most notable, in order to operate on a venv, all codepaths must go through Uv::venv, which will always check if the existing venv is compatbile. This is a safer, but more expensive path (since we at least always read the marker). If we are worried about this, we can introduce an assume_venv which will return UvWithVenv without checks.

[mitsuhiko]

I really like this. Something similar for toolchains is pending in #839. Definitely feels like a good direction.