feat(compilers): support split preprocessor inputs

[figtracer]

Summary

• allow compiler preprocessors to split one input into multiple compiler jobs
• allow multiple preprocessors to compose and postprocess outputs before artifacts are persisted
• discover top-level imports in bare Yul modules for dependency tracking

This is the compiler-framework prerequisite for foundry-rs/foundry#11059.

Testing

• cargo test -p foundry-compilers-core finds_yul_imports
• cargo test -p foundry-compilers --lib compile::project
• cargo clippy -p foundry-compilers-core -p foundry-compilers --lib -- -D warnings
• cargo +nightly fmt --all -- --check