Fix is_installed/get_installed_path crash on dotted package names

[yoarajota]

importlib.util.find_spec imports the leading component of a dotted module name to resolve it, and raises ModuleNotFoundError instead of returning None when that component isn't importable.

is_installed, get_installed_path, and package2module in mmengine/utils/package_utils.py call find_spec directly on the package name. Fine for a single-level name — find_spec('nonexistentpackage12345') returns None. Not fine for a dotted name whose leading segment isn't installed, e.g. a namespace package like zope.interface:

$ python -c "from mmengine.utils import is_installed; is_installed('zope.interface')"
Traceback (most recent call last):
  ...
ModuleNotFoundError: No module named 'zope'

is_installed should return False here — that's what the distribution() fallback exists for. get_installed_path should raise the PackageNotFoundError it documents, not a ModuleNotFoundError naming a truncated module that was never the argument.

Reachable through Config._get_cfg_path (mmengine/config/config.py:1263), which calls is_installed(package) to give a friendly error when an external config's scope package isn't installed.

Reproduced on current main (f8a8b1b) with Python 3.12.3, command above.

Fix: wrap the find_spec calls in is_installed and package2module with try/except ModuleNotFoundError, same handling as a None return. Added regression tests to tests/test_utils/test_package_utils.py covering both functions with a dotted name whose leading component doesn't exist.

pytest tests/test_utils/test_package_utils.py passes, 2/2. flake8 and isort are clean on the diff.

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[yoarajota]

@HAOCHENYE could you take a look when you have some time? The two failing checks (ci/circleci: lint and pr_stage_test) fail the same way on the other open PRs, like #1696 and #1698, so I believe they're unrelated to this change. Locally, pytest tests/test_utils/test_package_utils.py, flake8 and isort are clean.