refactor: dedicated struct for building source data

[erikgb]

The main motivation for this PR, is to get closer to a solution to #58, as the bundle controller cannot be used directly for this use case.

But I can identify a few more improvements in this:

• Better modularization, though far from perfect yet.
• Avoid calculating the bundle hash for each and every bundle target (which can be many).

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[cert-manager-prow]

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[erikgb]

/cc @inteon @SgtCoDFish