BF: install relpath semantics

[mih]

Fixes #3469

[yarikoptic]

A question

[yarikoptic]

That is what confuses me - why then this new function should not just replace the old one? When should I use old over overwatch if the new one?

[mih]

It always has to go. I just don't have the juice to do it all at once: #3778

[codecov]

Codecov Report

Merging #3777 into master will decrease coverage by 1.39%.
The diff coverage is 87.5%.

@@           Coverage Diff            @@
##           master   #3777     +/-   ##
========================================
- Coverage   71.19%   69.8%   -1.4%     
========================================
  Files         273     273             
  Lines       35973   35968      -5     
========================================
- Hits        25610   25106    -504     
- Misses      10363   10862    +499

Impacted Files	Coverage Δ
datalad/distribution/tests/test_install.py	99.6% <100%> (ø)	⬆️
datalad/distribution/install.py	46.6% <50%> (ø)	⬆️
...ad/metadata/extractors/frictionless_datapackage.py	27.08% <0%> (-68.75%)	⬇️
datalad/metadata/extractors/xmp.py	24.07% <0%> (-68.52%)	⬇️
datalad/metadata/extractors/datalad_rfc822.py	30.9% <0%> (-67.28%)	⬇️
datalad/metadata/extractors/exif.py	34.37% <0%> (-62.5%)	⬇️
datalad/metadata/extractors/audio.py	37.14% <0%> (-60%)	⬇️
datalad/metadata/extractors/datacite.py	31.25% <0%> (-58.34%)	⬇️
datalad/metadata/extractors/annex.py	35.48% <0%> (-58.07%)	⬇️
datalad/support/status.py	33.33% <0%> (-57.58%)	⬇️
... and 33 more

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