fix(imageloader): image loader should consider existing channels during add

[ecordell]

Description of the change:
This queries the database for existing channels before adding an image and replacing its graph.

Also, #213 moved the image loader to registry and renamed it, but didn't clean up the old image loader. I added the fix to the new location and cleaned up the old location.

Motivation for the change:
This fixes the regression reported in #258

In addition to the tests added in this PR, I also verified manually with the manifests in the report:

$ opm registry add -b quay.io/ecordell/brokenoperator:v1.0.0 -c docker
$ sqlite3 bundles.db "select * from channel;"
beta|test-operator|testoperator.v1.0.0

$ opm registry add -b quay.io/ecordell/brokenoperator:v1.0.0 -c docker
$ sqlite3 bundles.db "select * from channel;"
beta|test-operator|testoperator.v1.0.0
stable|test-operator|testoperator.v1.0.1

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[njhale]

Thanks! I think this looks good. Let's just get a second set of eyes to verify.

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ecordell, njhale

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[ecordell]

/retest

[kevinrizza]

/lgtm