Some fixes for AoT plugins (#124)

* Use `multi_value=False` for locally-defined properties

Use `multi_value=False` rather than `True` for AoT properties.
As Jonathan pointed out, multi-value properties only make sense
for filtering, given that the system may support only a subset of all
properties.  However, AoT properties are always supported, so there
is no real difference between a wheel built with multiple values
for a given property, and one built with just the highest value.

Signed-off-by: Michał Górny <[email protected]>

* Use a dedicated AoT plugin for testing

Use a dedicated AoT plugin rather than piggy-backing on a regular
plugin, where independent changes can trigger test failures.

Signed-off-by: Michał Górny <[email protected]>

* Make a random test property multi-value for better testing

Signed-off-by: Michał Górny <[email protected]>

* Validate that AoT plugins don't return multi-value features

Signed-off-by: Michał Górny <[email protected]>

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Signed-off-by: Michał Górny <[email protected]>

Add JSON Schema URL to variants.json

Fixes to regressions in 2197d01 (#71)

* Fix missing type hints

* Remove non-existing `clear_console` kwarg

* Fixes

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Co-authored-by: Jonathan Dekhtiar <[email protected]>