Collect preprocessed T2w image in collect_derivatives

[tsalo]

Closes #510. This adds a t2w_preproc query to the io_spec.json and an extra workaround for queries that start with t2w_ in collect_derivatives. I don't think this is the most elegant solution, so I'm happy to switch to another approach if one makes sense.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.82%. Comparing base (bf1df32) to head (93cb2ee).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
+ Coverage   70.61%   74.82%   +4.20%     
==========================================
  Files          25       25              
  Lines        2083     2085       +2     
  Branches      272      273       +1     
==========================================
+ Hits         1471     1560      +89     
+ Misses        553      456      -97     
- Partials       59       69      +10     

Flag	Coverage Δ
ds054	44.66% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tsalo]

One other idea: what about this

• Change preproc to preproc_t1w and t2w_preproc to preproc_t2w
• Change if not key.startswith('t2w_') to if not key.startswith('preproc_')
• Prepend anat_ instead of t1w_.

I think that's more readable for T2w-focused workflows like Nibabies.

EDIT: After looking through the workflows a bit more, I see that this require a heavy lift to make sure workflow inputs and outputs use consistent names.