2X faster startup, defer galaxy-* imports to when needed

[mr-c]

From 1.84user 0.18system 0:02.03elapsed to 0.71user 0.11system 0:00.83elapsed for cwltool --help

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.59%. Comparing base (380143e) to head (9fd4298).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cwltool/software_requirements.py	75.00%	2 Missing ⚠️
cwltool/main.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2155      +/-   ##
==========================================
+ Coverage   84.56%   84.59%   +0.03%     
==========================================
  Files          46       46              
  Lines        8389     8389              
  Branches     1970     1970              
==========================================
+ Hits         7094     7097       +3     
+ Misses        828      824       -4     
- Partials      467      468       +1     

☔ 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.