Fix rollup dependency removal

[tivac]

Fixes #441

What I originally thought was a tree-shaking bug turned out to be way more complicated.

So, here's what was going on.

• I changed the transform hook to check if a file was known to the Processor instance and then remove the file being transformed, all the files that depended on that file, and all the files it depended on.
• This change makes sense in the context of watching files for changes given how rollup doesn't tell us exactly what file changed so the tree needs to be aggressively pruned to avoid stale output.
• But that assumption was real bad in the case where an already-populated Processor instances was passed to the rollup plugin (like when using modular-css-svelte).
• Since that tree wasn't being re-processed the new aggressive removal logic would end up leaving holes in the tree due to the tree-walking order.

So, how to fix it? The answer for now is to support both flows!

If a file is already in the tree the code needs to check how many times it's run a build. If it's the first time do the light removal that only removes that file and the files that depend on it. If this is a watch-triggered multi-build then the aggressive removal step needs to be taken to also remove all the files the target file depends on.

phew

[codecov]

Codecov Report

Merging #442 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #442      +/-   ##
=========================================
+ Coverage   99.09%   99.1%   +<.01%     
=========================================
  Files          31      31              
  Lines         776     780       +4     
  Branches      119     120       +1     
=========================================
+ Hits          769     773       +4     
  Misses          7       7

Impacted Files	Coverage Δ
packages/rollup/rollup.js	100% <100%> (ø)	⬆️

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