Write per-crate query data in .sqlx directories unconditionally

[tamasfe]

I remember this being mentioned somewhere but I cannot find the issue/PR anymore. Found it in #1598, that issue aims at a complete rework of how the macros work, this one proposes smaller changes in the current one with much less work involved.

The idea would be that macros would populate .sqlx queries for offline use unconditionally and cargo sqlx prepare would become optional for cleanup or double-checking purposes. cargo sqlx prepare can be slow-ish for large workspaces and it's easy to forget running it before a commit unless a git hook is set.

If this is implemented, I would also remove the workspace-level .sqlx directory and let each crate manage its own queries, this would somewhat complicate prepare --check though.