Refactor loading of extensions: load all by default but allow for configuration

[yarikoptic]

It should

• make it easier for naive users to use DataLad -- typically they just install an extension and get extension functionality immediately exposed, e.g. like was reported in
  • failed to clone from demo.dataverse.org datalad-dataverse#302
  • TODO: check that actually this PR makes it not needed...
• retain capability to whitelist a list of extensions to consider via datalad.extensions.load configuration variable
  • an option is to deprecate it in favor of datalad.extensions.enable which would be more descriptive
• make it uniform across all points which consider extensions, not only for loading
  • or are there specific use-cases where you want to have extension functionality available but not "loaded"?

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 91.30%. Comparing base (fda2048) to head (e53242b).

Files	Patch %	Lines
datalad/support/entrypoints.py	81.81%	4 Missing ⚠️
datalad/local/wtf.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            maint    #7597      +/-   ##
==========================================
- Coverage   91.30%   91.30%   -0.01%     
==========================================
  Files         325      325              
  Lines       43459    43478      +19     
  Branches     5781     5786       +5     
==========================================
+ Hits        39682    39698      +16     
- Misses       3762     3765       +3     
  Partials       15       15              

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