Support nested actions

[Vampire]

The verification action should search all action YAML files in the repository and validate their typings against them accordingly.
Currently only the main action's typings are verified.

[krzema12]

Great spot. It's a bug. We should fix it with a major version bump because for some clients it may be a breaking change to add the missing validation.

We already have one action with nested actions that provide typings, which increases the urgency of this bug: https://github.com/Wandalen/wretry.action