ENH+BF: find_files to take list of topdirs, look for _bold only under sub-* directories

[yarikoptic]

Originally issue reported on
https://neurostars.org/t/heudicov-permission-denied-bids-derivatives-mriqc/16543
where heudiconv would crash while trying to create an _events.tsv file under derivatives/mriqc.

heudiconv should not care to look anywhere besides immediate subject directories, even if people
place derivatives direcly in the dataset into which they are also converting their data into
(making YODA unhappy).

TODOs

• verify on a sample case that there is no side effect (not sure if I would be up for composing a test)

[codecov]

Codecov Report

Merging #496 (2e1e1d3) into master (0e0f9e3) will increase coverage by 0.81%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   76.01%   76.82%   +0.81%     
==========================================
  Files          38       40       +2     
  Lines        3010     3094      +84     
==========================================
+ Hits         2288     2377      +89     
+ Misses        722      717       -5     

Impacted Files	Coverage Δ
heudiconv/bids.py	83.06% <100.00%> (ø)
heudiconv/parser.py	92.70% <100.00%> (+0.31%)	⬆️
heudiconv/tests/test_main.py	100.00% <0.00%> (ø)
heudiconv/tests/test_dicoms.py	100.00% <0.00%> (ø)
heudiconv/tests/test_convert.py	100.00% <0.00%> (ø)
heudiconv/tests/test_heuristics.py	100.00% <0.00%> (ø)
heudiconv/heuristics/bids_PhoenixReport.py	93.33% <0.00%> (ø)
heudiconv/heuristics/test_b0dwi_for_fmap.py	92.30% <0.00%> (ø)
heudiconv/utils.py	90.32% <0.00%> (+0.40%)	⬆️
heudiconv/dicoms.py	82.11% <0.00%> (+0.96%)	⬆️
... and 1 more

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[yarikoptic]

AFAIK should be ok to go