refactor: move fetching of source metadata to command dispatcher

[baszalmstra]

This PR replaces the code in the BuildContext to fetch source metadata with an implementation in the CommandDispatcher.

This involves:

• Checking out the source code of a dependency.
• Discovering the build backend to instantiate
• Instantiating the backend
  • Solving the build backend environment
• Calling into the build backend over RPC
• Caching the result.

I tested this locally against all the examples in examples/pixi-build (and added another one) and verified that locking, relocking, satisfiability and installing still functions as expected.

Some things that I changed:

• Build backends are always resolved to check for newer dependencies, this ensures that you always get the latest compatible version.
• Changed the location and names of several caches. This might invalidate previous caches.

[Hofer-Julian]

Code looks great, and it works as expected on my machine!

[ruben-arts]

I've been testing pixi build with it today, and didn't see anything unexpected. I've also just run all examples and there is no unexpected behavior. I feel confident merging this!