fix: alternate fix for dotted plugin_name

[tlambert03]

alternative to #233

@nclack, this implements option 1 from #233 (comment)

it populates Executable._plugin_name during the PluginManifest.__init__, and falls back to option 4 if nothing has been set (i.e. if it's a one-off contribution), and then lastly, to the old period split if nothing is found to be installed

[codecov]

Codecov Report

Merging #238 (efea475) into main (a853b4e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #238   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           37        37           
  Lines         2742      2753   +11     
=========================================
+ Hits          2742      2753   +11     

Impacted Files	Coverage Δ
npe2/manifest/contributions/_readers.py	100.00% <100.00%> (ø)
npe2/manifest/contributions/_sample_data.py	100.00% <100.00%> (ø)
npe2/manifest/contributions/_widgets.py	100.00% <100.00%> (ø)
npe2/manifest/contributions/_writers.py	100.00% <100.00%> (ø)
npe2/manifest/schema.py	100.00% <100.00%> (ø)
npe2/manifest/utils.py	100.00% <100.00%> (ø)

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[nclack]

Holy cow this is amazing.

[kmdouglass]

Thanks again @tlambert03 and @nclack !