High Memory Usage of built_value_generator to due memoization + keeping all SerializerSourceClass objects in memory

Hi, it's me again with issues that pop up due to ferry using built_value for serialization of generated graphql code, which can get quite big ;)

`built_value_generator` keeps a list of `SerializerSourceClass` for a `SerializerSourceLibrary`.

So all the `SerializerSourceClass` objects are kept in memory, and they use memoization to keep e.g. their `parsedLibrary` and `SerializerSourceField` `fields` cached.

This leads to very high memory usage in cases where there are a very high number of serializers.

For reference: https://github.com/gql-dart/ferry/issues/558#issuecomment-1854197907

This can probably be fixed with little changes to the code, e.g. be copying the currently active `SerializerSourceClass` into a new object which can is not held in a list after the code generation is done
(e.g. `sourceClasses.map((clazz) => clazz.rebuild((b) => b))`, so the objects with memoized fields are eligible for garbage collection after work is finished for that class.