import-trace: alias-based (importer) registry

This change implements a generic registry in order to clean up import-trace's
code - it's possible (and reasonable) to use the registry for pruners as well.
Importer now extends AliasedRegisterable; all importers have been adapted
to suit the interface/abstract methods.
Each AliasedRegisterable should have at least one alias (the class' name
is a sensible choice) but can have several. The first specified alias is
the class' prime alias which can be used e.g. to list all registered objects.

Change-Id: If6daa34edce35a3b0194e4ba67ed3b44b74a49b0

src/core/util/CMakeLists.txt

@@ -29,6 +29,9 @@ set(SRCS
  SynchronizedQueue.hpp
  WallclockTimer.cc
  WallclockTimer.hpp
+ AliasedRegistry.hpp
+ AliasedRegistry.cc
+ AliasedRegisterable.hpp
 
 )