Rebuilds shouldn't hit the file system to get the list of inputs

[jakemac53]

Today each rebuild will list the file system for all files. We should instead just keep an updated set of valid inputs, based on updates that are seen.

[natebosch]

This is fixed now. The only place we call AssetReader.findAssets is in build_definition.dart, and that only happens on the first load. Subsequent builds based on the file changes update the asset graph without listing from disk.