chore: rename salsa to ra_salsa

[davidbarsky]

Laying some groundwork to start before I import the new Salsa crate. Here's why:

1. As part of the migration, @darichey, @Wilfred, and I will create new Salsa equivalents of the existing databases/query groups. We'll get them to compile crate-by-crate.
2. Once we wrote all equivalents of all queries, we'd start to refactor usage sites of the vendored Salsa to use the new Salsa databases.
3. Starting porting usage sites of old Salsa to the new Salsa.
4. Remove the vendored ra_salsa; declare victory.