lang/rust-bootstrap: Unbreak makesum after d50822c3

It could not really work before since MASTER_SITES also changes
based on FLAVOR.

While here

- move distfiles to rust/bootstrap
- use per flavor MASTER_SITES groups to separate the files properly
- change distclean to also just work like makesum
- set DIST_SUBDIR to fix fetching the rustc tarball (was not a
  problem so far since lang/rust-bootstrap depends on lang/rust
  which fetches it before)