This commit refactors the internals of Cargo to no longer have a
singular `--target` flag (and singular `requested_target` kind throught)
but to instead have a list. The semantics of multiple `--target` flags
is to build the selected targets for each of the input `--target` flag
inputs.
For now this is gated behind `-Zmultitarget` as an unstable features,
since I'm not entirely sure this is the interface we want. In general
it'd be great if we had a way to simply specify `Unit` structures of
what to build on the CLI, but we're in general very far away from that,
so I figured that this is probably sufficient at least for testing for
now.
cc #8156
