Drop packageManager support & refactor pruning to be part of the copy filter

[malept]

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• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

See NEWS.md changes for summary.

Fixes #198.
Closes #515. (Made obsolete)
Closes #679. (Made obsolete)
Fixes #738.
Fixes #796.
Fixes #797.

[codecov]

Codecov Report

Merging #819 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #819   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         607    623   +16     
=====================================
+ Hits          607    623   +16

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

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