No need to introduce the LayeredCatalog anywhere it's not needed.
Simplify some more things about LayeredCatalog:
- remove unused containingCatalog link from localCatalog to
layeredCatalog
- because of that, simplify LayeredCatalog initialization to occur after
localCatalog (no more circular references required)
- drop some other dead LayeredCatalog methods
This reimplements functionality that had been removed as part of the
`isopack-cache` branch refactoring.
Information about package changes is encapsulated inside a
PackageMapDelta object on the ProjectContext. It is the responsibility
of the command that prepares the ProjectContext to choose to call
projectContext.packageMapDelta.displayOnConsole at the appropriate time
if it wishes to display changes.
Part of #3006.
Note that unlike the previous implementation, we show a 'downloading'
progress bar iff we actually download any packages. (In 1.0 we show a
'downloading' message while just deciding if we need to download
anything; on devel right now we never show 'downloading' since that was
confusing.)
