Delete PackageLoader, references to it in new code

References are still sitting around in code that hasn't been
transitioned yet.
Delete all references to PackageCache
Delete getLoadPathForPackage and _build
LocalCatalog has no builds
Stop fiddling with local package build IDs

We still prevent you from specifying a build ID in package.js, but we no
longer need it for internal rebuild-tracking purposes. (Maybe in the
future we can actually use it for tracking different builds of a
package?)
Move isopack-compiler back to compiler