The majority of asset reimport machinery is now held within UAssetImportData and FAssetImportInfo. This change allows us to remove the CachedAssetsBySourceFileName map from FAssetRegistry that was previously added to support auto reimport for 4.7.
The functionality of CachedAssetsBySourceFileName has been wrapped up in FAssetSourceFilenameCache, which sits on top of the asset registry in the editor.
This unification allows us to also consistently store the file timestamps and MD5 hashes of imported files. This subsequently allows auto-reimports to be ignored where a source content file has not actually changed (its MD5 is the same).
This addresses UETOOL-365 - Rework AssetRegistry changes to support auto-reimport
[CL 2567286 by Andrew Rodham in Main branch]
