Support --build-plan

[ehuss]

The --build-plan flag mostly works, but it does not capture the way the artifacts are copied to the sysroot as of rust-lang/cargo#7421. It also needs some tests, and probably some careful review.

It may be possible to just add the sysroot artifacts to the "links" list? That may be difficult to modify that list, though, as it currently uses a different mechanism to hard link the files.

Another possibility is to change the way Cargo hard links the sysroot artifacts. It is currently implemented in an unusual way in order to support pipelining between the last sysroot crate and the user's crate. This may not be a substantial benefit, so if pipelining was disabled between standard lib to the user's crate, a different approach may be taken (possibly as an OutputFile::hardlink?).

I'm also a bit uncertain about the future of build-plan in general.

[Ericson2314]

rust-lang/cargo#7614 Looks like --build-plan is out so this can be closed?