Fix the configuration cache support.

[nedtwigg]

2.2.0 was supposed to add support for configuration cache, but was actually broken. This PR fixes it, but at the following cost:

• a single input SVG can produce 0 to n output files
• because of this, our task has to maintain the mapping from input to output files manually, there isn't a guaranteed 1 -> 1
• previously, we did this with a locally mutable cache, which was always a risky choice
• now we have an InputFile mappingFileIn and OutputFile mappingFileOut, the last step of the task is to write mappingFileOut and copy it back to mappingFileIn
• this means that whenever the file topology changes (files added, removed, or changed in a way that affects the number of output files), we have to a full rerun of the task. Incremental only works when on pure modifications, no change in number of outputs.

Overall not a big deal. EDIT: See next comment.

[nedtwigg]

It actually is a big deal. It breaks the buildcache, which is more important most of the time, and adds a ton of complexity.

To do this right, we would need an incremental task that does .svg -> .tar, 1:1, where the tar would have each file's path and content in some temp dir. Then a separate non-cached, non-incremental task which untars each of those to the destination directory.

More complexity than it's worth, I think.

For now, I'm just ripping out the incremental part, which makes everything else much simpler anyway.