Avoid regenerating crates unnecessarily

Nobody has claimed this yet.

First of all, thanks for the awesome tool.
It really feels like an improvement over build scripts for code generation.

I use `cargo px` to generate ICU data for display purposes with [`icu_datagen`](https://docs.rs/icu_datagen/latest/icu_datagen/).

However, `cargo px` regenerates the data module every build, which means that dependent crates also get rebuilt on every run.
It would be nice if we could tell `cargo px` a set of "input files" and it only regenerates the crate when those files change.

For now, I work around that by running `cargo px` only to regenerate the crate and using `cargo` normally otherwise.
But long term, it would be nice to not have to keep the difference in mind.

Start with the cargo px generation flow and determine how it currently decides to regenerate the data crate. Define how a user-supplied set of input files is tracked, then verify that unchanged inputs skip regeneration while changed inputs rebuild the crate and its dependents as needed.

Written by the indexing model from the issue text.