Fixes 2 bugs:

A) Runningassetmap:compile gives you apublic/assets/importmap.json for fast importmap dumping on production. But, runningassetmap:compile again later would re-use theimportmap.json info to create the newimportmap.json... meaning it would never update after the first compile.

B) The importmap process generates 2 pieces of information: (A) the importmap and (B) which files from the importmap should be preloaded. When we useassetmap:compile, we need to dump both pieces of info. So, we now also dump animportmap.preload.json file. These are TWO files (and not just one) because we avoid parsingimportmap.json at runtime: we read the file and dump it straight out. We DO need to parse the "preload" file. So, I've kept them separate.

Blocked by#50219, which has some changes that will fix the tests here.

Cheers!