Rename --derivatives parameter to --datasets

[tsalo]

Closes none. Splits out a backward-incompatible change from #1265.

Changes proposed in this pull request

• Rename the --derivatives parameter to --datasets, to prepare to allow BIDS-Atlas datasets.
• Change xcp-d to XCP-D throughout the package.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.55%. Comparing base (b160183) to head (06d049b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xcp_d/cli/combineqc.py	0.00%	1 Missing ⚠️
xcp_d/utils/bids.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1276      +/-   ##
==========================================
- Coverage   78.56%   78.55%   -0.01%     
==========================================
  Files          59       59              
  Lines        7309     7312       +3     
  Branches     1104     1105       +1     
==========================================
+ Hits         5742     5744       +2     
  Misses       1279     1279              
- Partials      288      289       +1     

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