fix: resolve MPI linker group issues and propagate library search paths

[ijatinydv]

This PR resolves the linking failures reported in #1226, specifically for projects using external C++ dependencies like libtorch.

Key Fixes:

• MPI Grouping: Added the missing -Wl,--end-group in src/metapackage/fpm_meta_mpi.f90. Previously, the unclosed group caused the linker to incorrectly wrap subsequent libraries into the circular dependency search, leading to resolution errors.
• Flag Propagation: Added logic in src/fpm_targets.f90to extract -L search paths from c-flags and cxx-flags. This ensures these paths are actually passed to the Fortran linker during the executable link step, which was previously missing.

Testing:

• Added and verified a new internal helper extract_library_paths with various inputs (single/multiple paths, no paths, and trailing flags).
• Ran fpm test and verified standard builds to ensure no regressions in core dependency handling.

Closes #1226