UX: Log clone-candidate number and URLs

[adswa]

Previously, it was not obvious from which URL a subdataset clone
was attempted, leading to confusion in #6080.
This change adds a one-time log message at the debug level to
report the number of assembled clone candidates and their URLs.

fixes #6080.

PS: can I use % string interpolation with a list like I have done here?

[adswa]

oh wrong base branch...

[codecov]

Codecov Report

Merging #6092 (54085f5) into maint (b33cf83) will decrease coverage by 54.59%.
The diff coverage is 42.57%.

❗ Current head 54085f5 differs from pull request most recent head 86e729a. Consider uploading reports for the commit 86e729a to get more accurate results

@@             Coverage Diff             @@
##            maint    #6092       +/-   ##
===========================================
- Coverage   90.11%   35.52%   -54.60%     
===========================================
  Files         312      312               
  Lines       42217    42476      +259     
===========================================
- Hits        38045    15088    -22957     
- Misses       4172    27388    +23216     

Impacted Files	Coverage Δ
datalad/cmdline/common_args.py	100.00% <ø> (ø)
datalad/cmdline/helpers.py	36.54% <0.00%> (-34.56%)	⬇️
datalad/core/distributed/tests/test_push.py	98.72% <ø> (-0.22%)	⬇️
...ad/distributed/tests/test_create_sibling_gitlab.py	0.00% <0.00%> (-100.00%)	⬇️
...talad/distributed/tests/test_create_sibling_ria.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/distributed/tests/test_ria.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/distributed/tests/test_ria_basics.py	0.00% <ø> (-97.87%)	⬇️
datalad/distribution/remove.py	58.11% <0.00%> (-32.48%)	⬇️
datalad/interface/__init__.py	100.00% <ø> (ø)
datalad/interface/base.py	82.57% <ø> (-7.72%)	⬇️
... and 289 more

---

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[adswa]

I think the test failures are unrelated

======================================================================
ERROR: datalad.metadata.tests.test_extract_metadata.test_ds_extraction
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/appveyor/projects/datalad/datalad/tests/utils.py", line 742, in _wrap_with_tempfile
    return t(*(arg + (filename,)), **kw)
  File "/home/appveyor/projects/datalad/datalad/metadata/tests/test_extract_metadata.py", line 48, in test_ds_extraction
    import libxmp
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/__init__.py", line 50, in <module>
    from .core import XMPMeta, XMPIterator
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/core.py", line 50, in <module>
    from . import exempi as _cexempi
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/exempi.py", line 69, in <module>
    EXEMPI = _load_exempi()
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/exempi.py", line 60, in _load_exempi
    raise ExempiLoadError('Exempi library not found.')
libxmp.ExempiLoadError: Exempi library not found.
======================================================================
ERROR: datalad.metadata.tests.test_extract_metadata.test_file_extraction
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/appveyor/projects/datalad/datalad/tests/utils.py", line 742, in _wrap_with_tempfile
    return t(*(arg + (filename,)), **kw)
  File "/home/appveyor/projects/datalad/datalad/metadata/tests/test_extract_metadata.py", line 88, in test_file_extraction
    import libxmp
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/__init__.py", line 50, in <module>
    from .core import XMPMeta, XMPIterator
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/core.py", line 50, in <module>
    from . import exempi as _cexempi
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/exempi.py", line 69, in <module>
    EXEMPI = _load_exempi()
  File "/home/appveyor/dlvenv/lib/python3.8/site-packages/libxmp/exempi.py", line 60, in _load_exempi
    raise ExempiLoadError('Exempi library not found.')
libxmp.ExempiLoadError: Exempi library not found.

[yarikoptic]

Seems to be safe and shouldn't hurt. Thank you @adswa . Let's proceed