Don't import packages just for version compatibility check

[gjoseph92]

In #5695 (comment), we discovered that pandas was always being imported on Nanny workers because, to check version compatibility, all these modules are imported:

distributed/distributed/versions.py

Lines 15 to 29 in 9a4e0e2

	required_packages = [
	("dask", lambda p: p.__version__),
	("distributed", lambda p: p.__version__),
	("msgpack", lambda p: ".".join([str(v) for v in p.version])),
	("cloudpickle", lambda p: p.__version__),
	("tornado", lambda p: p.version),
	("toolz", lambda p: p.__version__),
	]
	optional_packages = [
	("numpy", lambda p: p.__version__),
	("pandas", lambda p: p.__version__),
	("lz4", lambda p: p.__version__),
	("blosc", lambda p: p.__version__),
	]

Instead of importing, we could use importlib.metadata.version to get the distribution's version number without importing the package. This could speed up worker startup time and reduce memory footprint a little.

cc @crusaderky