When a unit does not have Metadata, the computation of fingerprints
depends on reusing the same fingerprint file to detect if the
output needs to be rebuilt. The previous change that put each unit's
fingerprint into a separate directory was wrong, and broke change
detection in this case (for example, executables on MSVC).
Instead, use a different approach to deal with compiler output caching
by using the same naming convention as the fingerprint file itself.
Generalized `clippy_is_available` and renamed as `command_is_available`.
No checks in `ignores_failure_to_format_source`, it's not supposed to
use `rustfmt` even if it's available
This commit enables pipelined compilation by default in Cargo now that
the requisite support has been stablized in rust-lang/rust#62766. This
involved minor updates in a number of locations here and there, but
nothing of meat has changed from the original implementation (just
tweaks to how rustc is called).
