Refactor config default search path retrieval

[djhoese]

Port of dask/dask#8573 to donfig.

Authored by @jrbourbeau in the original dask PR. This removes a long deprecated path from the search list. A lot of the additional refactoring done by James in his PR is not needed here as donfig is already required to use a separate Config object for each new instance. So for this PR's tests the Config.paths property is considered public while it may not be considered that in dask. Lastly, I did not include James' last test:

def test_default_search_paths():
    # Ensure _get_paths() is used for default paths
    assert dask.config.paths == _get_paths()

as I'm not sure how I would structure donfig to do this. I could extract the path logic to another function, but it would depend on name of the config and the environment variable. At this time this logic is a little too interconnected. For now I'm leaving it.

[codecov]

Codecov Report

Merging #19 (322c802) into main (adbb8d1) will increase coverage by 0.60%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   93.81%   94.41%   +0.60%     
==========================================
  Files           4        4              
  Lines         582      609      +27     
==========================================
+ Hits          546      575      +29     
+ Misses         36       34       -2     

Flag	Coverage Δ
unittests	94.41% <100.00%> (+0.60%)	⬆️

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Impacted Files	Coverage Δ
donfig/config_obj.py	91.05% <ø> (+0.81%)	⬆️
donfig/tests/test_config.py	100.00% <100.00%> (ø)

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[jrbourbeau]

🎉

FYI there was one additional test-related change in dask/dask#8644 which was added after dask/dask#8573