[services] Remove path imports in tests

Summary:
https://linear.app/comm/issue/ENG-1300/[rust]-get-rid-of-import-paths-in-tests
Changes module structure to get rid of #[path=...] attributes. Doesn't change any runtime code, only mod and use statements (and removes a few now unnecessary #[allow(dead_code)]).

• Added mod.rs to backup and to blob to make them parent modules of the nested files
• Renamed lib/tools.rs to tools/mod.rs to make it a separate module

Test Plan: cargo test --no-run and check if it compiles

Reviewers: tomek, varun, jon, inka

Reviewed By: tomek, jon

Subscribers: ashoat, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D5171