Remove Boot-specifics from AetherGrapeEngine

With the goal of making AetherGrapeEngine generally useful with Groovy,
this commit removes any Boot specifics from it. Specifically, there
is now only a single default repository: Maven Central. The
Boot-specific Spring milestone and snapshot repositories are now added
via @GrabResolver annotations that are added using an ASTTransformation.

As part of this change, AetherGrapeEngine has also been updated to store
its repositories using a LinkedHashSet, this ensures that the same
repository is not used more than once while maintaining their ordering.