Refactor loader, resolver, release resolver out of the model package

[manno]

Don't merge before final-releases has been merged.

We want to vendor fissile from cf-operator (example).

This splits the fissile model into 4 smaller packages.

• LoadManifest is in separate loader package to reduce size when vendoring fissile
• downloading and opening releases is now in a separate releaseresolver package, which can be replaced when vendoring fissile
• extract manifest resolving/calculating into resolver package
• resolver package does most of the manifest validations now packages
• methods kept on structs, i.e. instance group, if possible
• kept load options in model package for simplicity
• added New*() functions
• kept tests intact
• totally avoids import cycles, had to move some tests into _test packages
• several structs and methods are public now, only because this change avoids to modify existing tests

Renamed and commented load release functions

• LoadReleases -> LoadReleasesFromDisk
• loadReleaseReferences -> downloadReleaseReferences

#161744671

[cfdreddbot]

:white_check_mark: Hey manno! The commit authors and yourself have already signed the CLA.

[jandubois]

Don't merge before final-releases has been merged.

I don't understand this request; there are no other pending PRs for fissile at this time, and the only final-releases PR I'm aware of is against the SCF repo: https://github.com/SUSE/scf/pull/1855

I'll add a WIP label to this PR; please remove that label when the PR is ready for review and merging!

[manno]

Thanks, turns out I was waiting for https://github.com/cloudfoundry-incubator/fissile/pull/449 Will rebase and remove the WIP label