fix(fmm-core): keep Rust crate imports in crate

[srobinson]

Summary

• Treat relative Rust manifest keys as Cargo workspace sources before generic dependency matching.
• Reuse the Rust path helper so crate:: imports stay scoped to the importing crate.
• Add a regression test covering same-basename modules in fmm-core and fmm-cli.

Test plan

• cargo run -q -p fmm -- generate
• cargo run -q -p fmm -- cycles --filter source --explain
• cargo test -q -p fmm-core dependency_matcher
• just check