fix(build): support xlings mcpp build dependencies

[Sunrisepeak]

Summary

• Support local/custom xlings-backed package indices during dependency install and lookup.
• Add compatibility handling for nested namespace package names such as mcpplibs.capi.lua.
• Preserve package-local include directory precedence through scanning, planning, Ninja rules, and compile_commands.json.
• Allow TOML arrays with a trailing comma.

Why

This preserves the mcpp-side fixes discovered while adding mcpp build support for openxlings/xlings. Without these changes, the xlings build fails on local path indices, project-scoped xlings install paths, nested namespace dependency de-duplication, and C package headers with colliding names such as common.h.

Validation

• git diff --check
• MCPP_HOME=/home/speak/.xlings/data/xpkgs/xim-x-mcpp/0.0.20 /home/speak/.xlings/data/xpkgs/xim-x-mcpp/0.0.30/bin/mcpp test -- --gtest_filter=Scanner.*:PmCompat.*:Toml.*
• MCPP_HOME=/home/speak/.xlings/data/xpkgs/xim-x-mcpp/0.0.20 /home/speak/.xlings/data/xpkgs/xim-x-mcpp/0.0.30/bin/mcpp build --print-fingerprint

Draft PR for review and preservation; not ready to merge until review findings are addressed.

[Sunrisepeak]

Follow-up pushed in a3a9951.

Implemented review follow-ups:

• centralized legacy flat dotted dependency-key parsing in mcpp.pm.compat, with the compatibility path marked for removal in mcpp 1.0.0;
• kept canonical nested dependency syntax working for both [dependencies] capi.lua = "0.0.3" and [dependencies.mcpplibs] capi.lua = "0.0.3";
• tightened custom-index first-use detection for both project data layouts and resolved local index paths relative to the project root.

Local verification:

• mcpp test -- --gtest_filter=Config.*:PmCompat.*:Manifest.Dependencies*:Toml.* passed;
• git diff --check passed;
• mcpp build --print-fingerprint passed with fingerprint afb76caa74c38c77;
• xlings e2e mcpp_build_xlings_test.sh passed using the newly built mcpp binary.