[tiny-ac]

Description:

Splits DistExecutor's remote compile into three steps, so a later PR can run the middle one in the sccache daemon while the client-side CLI runs the other two. That is what makes sccache-dist work
in client-side mode.

• prepare maps the compile's outputs to paths on the build server and builds its packagers. It needs the Compilation.
• dispatch runs the compile on the cluster and writes its outputs to their local paths. It needs only plain data and the packagers.
• finish passes the outputs to the OutputsRewriter and names the server in stderr if the remote compiler exited non-zero.

No behavior change: run_on_cluster calls the three steps in the order the code ran before, with the same logs, errors and output cleanup.

Changes

• src/compiler/remote/dist.rs: run_on_cluster split into prepare, dispatch and finish, with the DistRequest and DistResponse types they pass along.
• src/compiler/compiler.rs, src/compiler/remote/mod.rs: fixes from the RemoteExecutor trait PR's review. get_cached_or_compile asks the executor for its dist input options once and passes
  them to remote_or_local_compile. DistJob drops local_compiler: DistExecutor reads the compiler path from the local compile, and calls it compiler_path throughout.

Cost

One more String per distributed compile (out_pretty in DistRequest).

Validation

• fmt, and clippy with -D warnings.
• cargo test --lib --bins --tests: the dist tests in src/compiler/compiler.rs run all three steps, and the dist error test covers dispatch failing at each cluster call.
• cargo check --no-default-features --features dist-client.