feat: Introduce perma unstable --compile-time-deps option for cargo build

[ShoyuVanilla]

Resolves #14434

[rustbot]

r? @weihanglo

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[epage]

Figured I'd share my quick thoughts rather than make all wait on a deeper one.

[ShoyuVanilla]

I've pondered upon this more and decided not to introduce new UserIntent/CompileMode and I'll re-request you for reviews once I finished it. Sorry for bothering you

[ShoyuVanilla]

Oh, test failure looks strange 🤔 Maybe something related to Sized changed in rustc nightly?

 test failed running `/Users/runner/work/cargo/cargo/target/debug/cargo build --lib --target custom-target.json -v`
error: process exited with code 101 (expected 0)
--- stdout

--- stderr
   Compiling foo v0.0.1 (/Users/runner/work/cargo/cargo/target/tmp/cit/t1384/foo)
     Running `rustc --crate-name foo --edition=2015 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=d4fb0c790b31c5ca -C extra-filename=-cf2ab74374a2d944 --out-dir /Users/runner/work/cargo/cargo/target/tmp/cit/t1384/foo/target/custom-target/debug/deps --target /Users/runner/work/cargo/cargo/target/tmp/cit/t1384/foo/custom-target.json -L dependency=/Users/runner/work/cargo/cargo/target/tmp/cit/t1384/foo/target/custom-target/debug/deps -L dependency=/Users/runner/work/cargo/cargo/target/tmp/cit/t1384/foo/target/debug/deps`
error: requires `meta_sized` lang_item
  --> src/lib.rs:9:1
   |
9  | / pub trait Sized {
10 | |     // Empty.
11 | | }
   | |_^

[ShoyuVanilla]

I filed a new issue addressing this CI failure #15678

[epage]

Thanks!

[ShoyuVanilla]

Rebased on master to resolve the CI failure