gh-138122: Move local imports to module level in sampling profiler

[pablogsal]

The sampling profiler code had numerous imports placed inside functions
rather than at module level. While deferred imports can reduce startup
time for rarely-used code paths, these imports were in functions called
during normal profiler operation, adding repeated import overhead.

Moving imports to module level follows Python best practices for code
that runs frequently. This makes import dependencies explicit at file
scope and eliminates per-call import lookup costs. The test files also
had redundant local imports that duplicated module-level imports.

• Issue: Implement PEP 799 – A dedicated profiling package for organizing Python profiling tools #138122