Warn and discourage lazy.load of subpackages

[dschult]

The importlib.find_spec tool automatically (and eagerly) imports packages when subpackages are asked to be found. The older find_loader did not do that, but it is deprecated and will be removed in python v3.12. While it might be possible to make the load feature work using manually build ModuleSpecs, the payoff is unlikely to be worth the convoluted errors obtained when something goes awry. It would/will be much more effective for people to encourage the package to attach the subpackages so they can be lazily loaded.

That said, packages can still be lazily loaded. And even subpackages can be lazily loaded though this causes the package to be eagerly imported. So there might be some use for this load function. This potential benefit might be enough reason to keep the function in this library. But I can also see that removing the function altogether would be reasonable too.

With that in mind, this PR changes the function to raise a RuntimeWarning when the input is a subpackage (has "." in the name of the package). It also changes the doc_strings to discourage the use of lazy.load(sub.package). And it adds a test to check for the warning, and to verify that the return value of load with subpackage input continues to provide an eagerly imported package with a lazily loaded subpackage.

I think this is better than leaving load unchanged -- some of the docs even suggested using it for subpackages.

But, is this better than removing load?

[codecov-commenter]

Codecov Report

Merging #57 (42ac558) into main (a6188ac) will increase coverage by 3.42%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   90.53%   93.95%   +3.42%     
==========================================
  Files           4        4              
  Lines         169      182      +13     
==========================================
+ Hits          153      171      +18     
+ Misses         16       11       -5     

Impacted Files	Coverage Δ
lazy_loader/__init__.py	91.86% <100.00%> (+6.49%)	:arrow_up:
lazy_loader/tests/test_lazy_loader.py	96.73% <100.00%> (+0.35%)	:arrow_up:

[jarrodmillman]

LGTM. We can always deprecate (and later remove) load in a subsequent PR if we want.

[stefanv]

Thanks you @dschult. It's also worth referencing https://github.com/scientific-python/lazy_loader/issues/55.