Implement external build plan

[Xanewok]

From the commit description:

This adds a BuildGraph trait which aims to abstract away the API for
a build plan and it implements it for the in-process Cargo build plan
as well as the new, external one (for cargo build --build-plan
format).
In addition to that, since save-analysis (since rls-data 0.18.1)
includes the invocation used to compile a given crate, we try to
recreate the external build plan from the passed save-analysis files
via build_command config option.

This is squashed into a single commit not to miss anything during the rebase and because my branch diverged from head a bit and kept going back and forth; hopefully this will be legible.

TL;DR:

• Moved plan.rs -> cargo_plan.rs
• Added external build plan format (like in cargo build --build-plan)
• Added and implemented BuildGraph trait for both plans
• Recreate the build plan from invocations in save-analysis (for build_command case)

r? @nrc

[nrc]

Could you update Clippy so this runs tests on CI please?

[nrc]

I have not done the deepest review, but I think this all looks good. It would be good if you could do a bunch of manual testing to ensure we keep working on different kinds of projects in different situations - we're close enough to the edition release that I'd hate to introduce new bugs.

[Xanewok]

Will do, thanks for taking a look!

I kept the old code path mostly intact with exception of CargoPlan::prepare_work - previously it was passed needs_rebuild: bool and manifest_path: &Path, both used to possibly recreate PackageMap and return a request to run Cargo.
Since the Cargo build plan is always initialized for a cargo() run, I changed the lazy PackageMap loading to always do it upfront but it seems like an okay thing to do, since whenever we request a build with BuildPriority::Cargo (explicit rebuild or build dir changed, we set needs_rebuild) we always end up running cargo(), thus always regenerating package map if needed.

[Xanewok]

error[E0061]: this function takes 6 parameters but 7 parameters were supplied
   --> src/build/rustc.rs:296:13
    |
296 | /             save::process_crate(
297 | |                 state.tcx.expect("missing tcx"),
298 | |                 state.expanded_crate.expect("missing crate"),
299 | |                 state.analysis.expect("missing analysis"),
...   |
309 | |                 },
310 | |             );
    | |_____________^ expected 6 parameters
error: aborting due to previous error

Blocked on rust-lang/rust#54356