ENH: addurl - store original urlfile, not resolved full path

[yarikoptic]

Rationale - to make commit message "portable" happen someone needs to
redo it. Ideally the urlfile should be either "-" or relative within dataset
path. I decided to not inforce it but merely store the original specification
of the user.

Also decided to just adjust/reuse existing test to not add more time to setup
a dedicated one

NB positioned against master since changes the record/behavior, unless there is consensus that it is appropriate for maint ;)

[codecov]

Codecov Report

Merging #5091 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5091   +/-   ##
=======================================
  Coverage   89.82%   89.82%           
=======================================
  Files         293      293           
  Lines       41272    41274    +2     
=======================================
+ Hits        37073    37075    +2     
  Misses       4199     4199           

Impacted Files	Coverage Δ
datalad/plugin/addurls.py	99.72% <ø> (ø)
datalad/plugin/tests/test_addurls.py	100.00% <100.00%> (ø)

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