Prevent incorrect ImportAware metdata injection

Update ImportRegistry to track all import registrations that occur
against an importing class (rather than just keeping the last). In
addition, prune imported classes from the registry when a configuration
class is removed during the REGISTER_BEAN ConfigurationPhase.

This update prevents incorrect metadata from being injected into an
ImportAware class which is imported twice by different configurations
classes (when one of the configuration classes will be ultimately skipped
due to a @Condition).

Issue: SPR-12128