delete dead fields
refactor: move pluginProviderPackageMap in Isopack

This also un-breaks soft refresh for troposphere packages:
previousIsopack was accidentally being set to an
{isopack,pluginProviderPackageMap} object, and so the "can we reuse the
previous one" check was never actually passing.
Implement soft refresh.

Fixes #3213.

Also ameliorates the memory leak of parsedSourceMaps in
files.runJavaScript (because now we don't reload unchanged plugins).
Fix safe-pathwatcher memory leak