FIX: Fetch templates during workflow construction

[mgxd]

This addresses nipreps/fmriprep#3240, a few changes were necessary:

• Fetch templates during workflow creation - without this, I was still observing race conditions causing the T1w/brain mask files to be empty (not yet gotten through templateflow.api.get())
• Add sloppy parameter to init_template_iterator_wf, since the T1w/brain mask's resolution (used in SpatialNormalization) depends on this parameter as well.

I stripped out most of the logic in TemplateFlowSelect to avoid duplicating the behavior, since it is essentially linked.

[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.59%. Comparing base (2355035) to head (9f13a86).
Report is 5 commits behind head on master.

Files	Patch %	Lines
smriprep/interfaces/templateflow.py	92.00%	2 Missing ⚠️
smriprep/workflows/outputs.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #418       +/-   ##
===========================================
+ Coverage   62.91%   78.59%   +15.67%     
===========================================
  Files          24       30        +6     
  Lines        1912     2088      +176     
  Branches      242      248        +6     
===========================================
+ Hits         1203     1641      +438     
+ Misses        676      415      -261     
+ Partials       33       32        -1     

Flag	Coverage Δ
ds005	∅ <ø> (?)
ds054	46.28% <67.56%> (?)
pytest	68.29% <89.18%> (?)

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