fix: allow multiple entities to point to the same derived entity

[grezle]

Enables multiple entities to be @derivedFrom the same @entity by:

• packing the TypeDefinition.name into the self.derived tuple vector in src/context.rs
• iterating on the vector for a match before carrying out any derived entity lookups

Closes: #333

Updates the tests to match the altered definition

[georg-getz]

Thanks for the PR :)