Add basic C++ path mapping support

[fmeum]

Basic support for path mapping for CppCompile actions is added by wiring up PathMapper with:

• structured path variables for files and include paths (Artifact and NestedSet<PathFragment>)
• inputs/outputs via Spawn#getPathMapper()
• header discovery

Also turns external_include_paths into a structured variable, which was missed in #22463.

The following features are known to be unsupported for now:

• layering_check (requires rewriting paths to and in module maps)
• source tree artifacts (requires wiring up PathMapper in CppCompileActionTemplate)
• location expanded paths to generated files such as sanitizer suppression lists (requires heuristically rewriting paths in user_compile_flags)

These limitations will be lifted in follow-up PRs.

Work towards #6526

RELNOTES: Experimental support for path mapping CppCompile actions can be enabled via --modify_execution_info=CppCompile=+supports-path-mapping.

[fmeum]

@comius This test should serve as a representative example of what path mapping can do, but please let me know if there is any additional context I can provide to help you with the review.

[gregestren]

Sanity check: what's the story with debug symbols?

[fmeum]

Sanity check: what's the story with debug symbols?

At this point users need to opt in explicitly via --modify_execution_info and can simply choose not to do so when building with -c dbg. Long term we could either implement this behavior in rule logic or switch to a necessarily more complicated path mapping scheme based on content hashes or action hashes, but that would be very involved.