Handle already published workspace packages

[blink1073]

Do not error when a given npm package exists if there are multiple npm packages. This covers the case of independently versioned lerna repositories where only a portion of the packages are being updated.

[codecov-commenter]

Codecov Report

Merging #49 (2149097) into master (9129099) will decrease coverage by 0.23%.
The diff coverage is 80.55%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   88.51%   88.27%   -0.24%     
==========================================
  Files          15       15              
  Lines        1715     1740      +25     
  Branches      205      211       +6     
==========================================
+ Hits         1518     1536      +18     
- Misses        122      126       +4     
- Partials       75       78       +3     

Impacted Files	Coverage Δ
jupyter_releaser/lib.py	75.29% <60.00%> (-1.07%)	⬇️
jupyter_releaser/tests/test_cli.py	97.64% <91.66%> (-0.20%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/npm.py	80.91% <100.00%> (+0.29%)	⬆️
jupyter_releaser/util.py	85.00% <100.00%> (+0.10%)	⬆️

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