Refactor “get cached image” logic to a separate package in envbuilder

[mafredri]

The goal of this refactor is to allow the "get cached image" logic to be imported by another tool (the Terraform provider for envbuilder).

This can be done as a separate task (prereq) or in conjunction with the provider implementation.

Part of Envbuilder Terraform Provider RFC.

[johnstcn]

We'll want to make sure that the refactored code at worst does not leave any files lying around after execution, and at best can function with only a tmpfs and read-only rootfs.

[johnstcn]

This is done!