Provide extensions points so that DHF can avoid replacing tasks

[rjrudin]

DHF 5.x results in Gradle warnings due to some tasks being replaced. And the warnings note that task replacement may not be allowed in Gradle 6. So the following 3 tasks need to be reworked so that DHF can modify their behavior without replacing them:

• mlUpdateIndexes - should depend on an instance of an interface, and DHF can replace that with its own implementation (the interface could just be a Command object)
• mlClearModulesDatabase - same thing, should depend on an instance of Command
• mlWatch - change afterModulesLoader into an interface that DHF can provide an implementation of Consumer or some similar callback interface