Make sure HPO default lookup is based on class for losses

[cthoyt]

This PR uses the internal PyKEEN loss class hierarchy to make it possible to more elegantly define the HPO defaults, as in several other modules like models.

[codecov]

Codecov Report

Merging #111 into master will decrease coverage by 0.06%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   69.43%   69.36%   -0.07%     
==========================================
  Files          91       91              
  Lines        5562     5566       +4     
  Branches      682      680       -2     
==========================================
- Hits         3862     3861       -1     
- Misses       1501     1506       +5     
  Partials      199      199              

Impacted Files	Coverage Δ
src/pykeen/hpo/hpo.py	42.18% <16.66%> (-0.79%)	⬇️
src/pykeen/losses.py	98.66% <100.00%> (-0.02%)	⬇️

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