fix(load): shouldn't clear bundle/csv field from non-default heads

[ecordell]

Description of the change:
Updated the method that clears out unneeded metadata to avoid clearing any channel heads.

Also added a test case that would fail before this change, and helpers for enforcing invariants in our tests (started with two: all channel heads should have csv/bundle fields, and if an operator does not have a bundlepath, it must have a csv/bundle field).

Fixes #261

Motivation for the change:
Previously, this would clear out the csv/bundle field of all non-default channel heads (in addition to interior nodes). But non-default channel heads are used by OLM to display differences between channels to users, so we should retain that metadata.

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

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ecordell

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[kevinrizza]

/lgtm