c++modules: Restore use of local target for `import std`

Restore use of a project-local `STATIC` library to provide the stdlib's
module initialization symbols, as CMake 4.2's experimental support did.

In commit fa10dc6c22 (Experimental/CXXModules: Implement EcoStd Module
Metadata parser, 2025-10-21, v4.3.0-rc1~483^2) we switched to an
imported target to provide the BMI under the assumption that stdlib
implementations provide the symbols themselves, but they don't yet.

While refactoring the module metadata parser's properties logic, also
fix a relative path issue for local arguments.

Fixes: #27426, #27626, #27635