Every classes that's compiled by Kotlin is annotated with
kotlin.Metadata. The attributes of this annotation always differ so
if they are used in the cache key, context caching will effectively
be disabled.

This commit updates the key used by ImportsContextCustomizer to
ignore the kotlin.Metadata annotation. Additionally, to align with
with Java where annotations in java.lang.annotation are ignored,
annotations in kotlin.annotation are also ignored.

Closes gh-7101