refactor(utilities): rename utilities.lazy to utilities.importlib #2747

The owner asked in session to rename pcapkit.utilities.lazy, added by #2735 for #2729, to pcapkit.utilities.importlib or simply imp.

Taking importlib. It follows the stdlib-domain precedent set by pcapkit.utilities.pickle (#2722). imp is the name of the stdlib module removed in Python 3.12, so reusing it invites confusion. Say if you would rather have imp.

Scope

• Move pcapkit/utilities/lazy.py to pcapkit/utilities/importlib.py. lazy_exports keeps its name.
• Update every import site in the pcapkit.const packages, plus the tests.
• Rename docs/source/pcapkit/utilities/lazy.rst to importlib.rst, and update its toctree entry and every :mod: / :func: reference, including const-vendor-pairing.rst.
• utilities.lazy was never released, so the old path gets no re-export, no deprecation note and no mention anywhere.
• The protocols follow-up perf(protocols): import the protocol, schema and data packages lazily #2730 will use the new name.