Salsify the crate graph

[ChayimFriedman2]

I.e. make it not one giant input but multiple, for incrementality and decreased memory usage for Salsa 3 reasons.

This means that changing the metadata of a crate (e.g. adding a dependency) invalidates only it.

Draft for now because I still need to review the after-rebase changes.

[ChayimFriedman2]

Okay, this is now ready for review.

There are three big changes in this PR (I should've probably split them to commits, but now it is too late):

1. Changing the crate graph to be incremental. The changes are mostly in base-db, and also spread in a lot of small changes all over the codebase where we access crate metadata.
2. Splitting the "local" def map out of the def map, to allow for better incrementality for things like the extern prelude. This changes mostly in hir-def, in nameres.
3. Loading proc macros and build scripts and only then setting the crate graph, to avoid invalidations. This change is in the rust-analyzer crate.