Decompose experimental install resolver

[tolgaergin]

Summary

• move experimental install resolver graph construction into resolve/graph.rs
• move ambient peer planning and fetch scheduling into resolver submodules
• extract the old inline resolver unit tests into resolver test modules with shared fixtures

Local checks

• cargo fmt --check
• cargo check -p lpm-cli --locked
• cargo clippy -p lpm-cli --all-targets --locked -- -D warnings
• cargo test -p lpm-cli -- install::resolve --test-threads=1
• cargo test -p lpm-cli -- install:: --test-threads=1
• cargo build --workspace --locked
• cargo nextest run --locked --workspace --exclude lpm-workflows --exclude lpm-cli --no-fail-fast --status-level slow --final-status-level fail
• cargo test --locked -p lpm-cli --bin lpm-rs -- --test-threads=1
• cargo nextest run --locked -p lpm-cli -E 'not binary_id(/bin/lpm-rs/)' --no-fail-fast --status-level slow --final-status-level fail