import remotes use ContentIdentifier for getting and checking content

import remotes use ContentIdentifier for getting and checking content

This is better than using the equivilant actions for export remotes,
especially for getting content, since the ContentIdentifier checking
means we can be sure (enough) that the content is valid to not force
verification of content. Which allows getting keys of types that cannot
be verified.

Also, reorganized the internals of adjustExportImport which was becoming
very hard to follow. Now it's clear what each method does in each case.