fix(build): cache custom-index dependency objects

[Sunrisepeak]

Summary

• skip first-use xlings update for local-path custom indices that are already resolved directly from disk
• store dependency BMI cache entries under the dependency namespace/index instead of always using the default index
• preserve object paths relative to obj/ when populating dependency BMI cache so collision subdirectories round-trip correctly
• add an e2e regression covering a local custom index dependency with duplicate object basenames

Verification

• mcpp build
• MCPP=target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp bash tests/e2e/49_bmi_cache_nested_custom_index.sh
• MCPP_HOME=<temp pointing at complete installed mcpp registry> target/x86_64-linux-gnu/4d24c8b57fdbbbb4/bin/mcpp test -- --gtest_filter=BmiCache.*

Note: the normal local ~/.mcpp/registry/data/xpkgs/xim-x-binutils/2.42 is incomplete on this machine (only .xpkg.lua, no bin/as), so the filtered unit run used a temporary MCPP_HOME backed by the complete installed mcpp registry.