Refactor NewImageSource to add a manifest type abstraction

Currently, NewImageSource creates a Docker schema2 manifest and an OCI
manifest at the same time. This precludes functionality that isn't
supported by both manifest types, for example zstd compression.
Refactoring this to create only the desired manifest type solves this
and also cleans up the code by separating manifest-type-specific code
into distinct implementations of a "manifest builder".

See discussion in containers#5452.

Author: Aaron Lehmann