Prune user-namespaced modules

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (not applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Fixes #888.

I've tested with the minimal testcase in the related bug.

[codecov]

Codecov Report

Merging #889 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #889   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         616    617    +1     
=====================================
+ Hits          616    617    +1

Impacted Files	Coverage Δ
prune.js	100% <100%> (ø)	⬆️

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