The proposed solution doesn't work:
The ImportStepRegistry is not only used for imports - it is also used
for creating new 'import_steps.xml' files for exports.
Exported profiles are always base profiles, they have to specify *all*
import handlers used for creating the site.
I can see two ways to specify available import steps:
1.) 'import_steps.xml' is used as an index of available import steps.
That's the way it was used before extension profile support was added to
CMFSetup. It requires to maintain a lot of redundant information and a
way to identify and remove duplicate steps for exports.
2.) The files in the profile determine which import steps are available.
This is the current behavior, but further changes would make this more
clear: Getting rid of 'import_steps.xml', using a global handler
registry instead. And adding new code that checks if a step is available
without actually running it.
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