Clarify confusing INFO log message from get() on dataset installation

[mih]

The confusion arises from a message that announces the installation
of a (sub)dataset, even when the dataset is already present.

This change rewords the message to be more valid in this case.

Fixes #6862

[bpoldrack]

Issue remains a bit weird in the toplevel case, since this was ensured already and get would fail before otherwise, but I agree - this wording removes the confusion about the message itself.

[mih]

Thx for the review. I think all these log messages are a relic, and should be implemented properly as progress reporting. But I have no interest in working on this.

[codecov]

Codecov Report

Merging #6871 (a178c73) into maint (cefecae) will increase coverage by 0.97%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            maint    #6871      +/-   ##
==========================================
+ Coverage   90.25%   91.23%   +0.97%     
==========================================
  Files         354      354              
  Lines       46113    46127      +14     
==========================================
+ Hits        41621    42083     +462     
+ Misses       4492     4044     -448     

Impacted Files	Coverage Δ
datalad/distribution/get.py	96.76% <ø> (+0.40%)	⬆️
datalad/customremotes/tests/test_datalad.py	95.34% <0.00%> (-2.28%)	⬇️
datalad/distribution/create_sibling.py	69.80% <0.00%> (-0.20%)	⬇️
datalad/cli/common_args.py	100.00% <0.00%> (ø)
datalad/tests/test_dochelpers.py	100.00% <0.00%> (ø)
datalad/support/tests/test_network.py	100.00% <0.00%> (ø)
datalad/tests/utils_pytest.py	89.73% <0.00%> (+0.01%)	⬆️
datalad/utils.py	88.17% <0.00%> (+0.01%)	⬆️
datalad/__init__.py	98.00% <0.00%> (+0.04%)	⬆️
datalad/support/tests/test_locking.py	95.87% <0.00%> (+0.04%)	⬆️
... and 15 more

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