refactor: organize Rust code into crates and add new Python bindings

[wolfv]

This PR refactors the entirety of the rattler-build code base to make it much more modular.

A lot of the functionality has moved into specialized crates, such as:

• rattler-build-source-cache: contains code to checkout code from Git or URLs and cache it locally on disk
• rattler-build-recipe-parser: contains a revamped parser that now fails on undefined variables, even better used vars tracking, ... It also features a type-safe stage0 and stage1 recipe format for proper rendering stages.
• rattler-build-jinja: common settings for the Jinja execution
• rattler-build-script: The build script execution engine
• rattler-build-variant-config: load variant configurations from files and perform variant arithmetic
• and more...!

The refactor also comes with totally revamped Python bindings that give much more control and offer great flexibility when it comes to building packages.

While we are merging this code now, it is still considered beta and we are fixing a few last parser differences / errors (by running against a large amount of recipes-in-the-wild over at https://github.com/prefix-dev/rattler-build-parser-tests.