single-process: handle step-level req collisions

[mr-c]

Fixes #1602

[codecov]

Codecov Report

Merging #1603 (f149c3d) into main (0c39dcc) will increase coverage by 1.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1603      +/-   ##
==========================================
+ Coverage   65.09%   66.30%   +1.20%     
==========================================
  Files          93       93              
  Lines       16487    16503      +16     
  Branches     4370     4378       +8     
==========================================
+ Hits        10733    10942     +209     
+ Misses       4580     4408     -172     
+ Partials     1174     1153      -21     

Impacted Files	Coverage Δ
cwltool/workflow.py	85.65% <100.00%> (+0.47%)	⬆️
docker.py	49.37% <0.00%> (+0.41%)	⬆️
executors.py	65.31% <0.00%> (+0.45%)	⬆️
main.py	31.90% <0.00%> (+0.53%)	⬆️
process.py	70.18% <0.00%> (+0.91%)	⬆️
job.py	62.52% <0.00%> (+1.18%)	⬆️
cwltool/subgraph.py	80.24% <0.00%> (+1.23%)	⬆️
singularity.py	52.33% <0.00%> (+2.80%)	⬆️
command_line_tool.py	66.43% <0.00%> (+3.43%)	⬆️
builder.py	83.04% <0.00%> (+3.50%)	⬆️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0c39dcc...f149c3d. Read the comment docs.