public interface for module loading

[jurgenvinju]

• hide the generated public constructors with a @deprecated "for internal use only" tag (because the contract with the $ModuleStore can be violated with manual usage of these constructors)
• remove unused fields pcfg and rascalSearchPath from the RascalExecutionContext
• generate a safe static $load method into all classes and interfaces to replace the old usage of constructors (completely and safely hides the moduleStore below the hood).
• make certain internal methods of RascalExecutionContext @deprecated "for internal use only"

For example:

• $ParserGenerator.$load(rex) will return a $ParserGenerator_$I implementation.
• Also $ParserGenerator_$I.$load(rex) does the same for convenience' sake.
• repeated calls to $load will return the previously loaded module
• $load on a previously imported module from the same rex will return the module loaded
  earlier via the import mechamism: $List.load(rex)
• A new rex means new object instances which are initialized from scratch.
• Calling $load for different modules in the same import/extend hierarchy
  in different orders has no influence on the resulting module references. This is due to how the ModuleStore works.
  With one exception: the values of globals may be influenced in the presence of side-effects.

[jurgenvinju]

@PaulKlint wasn't this one merged already?

[PaulKlint]

@jurgenvinju: no it was not yet merged for statbility reasons. Will do asap.

[PaulKlint]

@jurgenvinju: you probably did not test this, does not work. Fixing it now.

[PaulKlint]

I have reverted the merge of this code since it does not seem to work. More testing needed.