Remove addition of Prelude import to AST

Currently, during compilation the AST of a module is extended with a "virtual" import of the Prelude module unless the Prelude is explicitly imported or the import is deactivated by the NoImplicitPrelude language extension.

While the consideration of the Prelude is important for compilation, the addition to the AST can lead to problems since the AST no longer corresponds to the source program. Thus, this transformation should at least be deferred to the transformation phases.

Involved modules are:

• Modules: Contains the function importPrelude which should be removed
• Interfaces: Contains the function loadInterfaces which then has to load the Prelude in addition
• Imports: Contains the functions importModules and importInterfaces which then have to load the Prelude in addition