Imported plan's enabled key is ignored by tmt run

[happz]

Import a plan with enabled: false, and tmt run will happily run it. As long as the importing plan itself is enabled, filtering of enabled/disabled plans happen before the import, and it's not repeated.

[lukaszachy]

Hopefully simpler reproducer, extract into /tmt so /tmp/reproducer directory exists:
in /tmp/reproducer/git1:

	tmt plan show	tmt -c foo=bar plan show
/plan/no	enabled: false	enabled: false
/plan/no_in_adjust	enabled: true	enabled: false
/plan/yes	enabled: true	enabled: true
/plan/yes_in_adjust	enabled: false	enabled: true

/tmp/reproducer/git2 contains plans which import the above so they should follow same enabled/disabled depending on context.

Note we have tmt plans --enabled|--disabled filters which should work too.

https://drive.google.com/file/d/1rhiOZukizgC5IntTPIMUs4ZRoPsxQjRz/view?usp=sharing