lang/rust-bootstrap: Unbreak makesum after d50822c34fe1

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)