Refactor Module.getExtensions

Module.getExtensions() is fully unit tested since

• https://github.com/openmrs/openmrs-core/commit/61a1b3ef5ce0ef2b2612c2c7b54f4c5d3fd3becf
• https://github.com/openmrs/openmrs-core/commit/d33ab88d966193833792047d7bb91796107da9de

I suggest to improve readability of the code by

• extracting some private methods for
• removing nesting via guard clauses, removing unreachable conditions

and

• improve javadocs
• make logging statements consistent