Remove `atlas_install_headers(DIR)` directive if the directory `DIR`
does not even exist, i.e. there are no headers to install.

The Gaudi PluginSvc has been refactored and modernizes.
See gaudi/Gaudi!709

As well as some changes to macros that directly call on
certain features of the PluginSvc itself, other code that
has been impacted is as follows:

 - destructors of Converters must be public
 - template arguments of ServiceHandles and ToolHandles cannot always be forward defined and the associated header files may need to be explicitly included


Former-commit-id: 83042516

major changes:

 - removal of DECLARE_FACTORY_ENTRIES
 - DECLARE_CONVERTER_FACTORY -> DECLARE_CONVERTER
 - DECLARE_TOOL_FACTORY -> DECLARE_COMPONENT
 - DECLARE_SERVICE_FACTORY -> DECLARE_COMPONENT
 - DECLARE_ALGORITHM_FACTORY -> DECLARE_COMPONENT
 - constructor of Converters is public, not protected
 - remove #include "GaudiKernel/CnvFactory.h"
 - remove #include "GaudiKernel/ToolFactory.h"
 - remove #include "GaudiKernel/AlgFactory.h"
 - remove #include "GaudiKernel/AudFactory.h"
 - remove #include "DeclareFactoryEntries.h"


Former-commit-id: fd7a40f5

This reverts commit cdf718bf [formerly 9d67f5b8].


Former-commit-id: 008b62d0

This reverts commit 9156e32a [formerly f73d17d3], reversing
changes made to 27d106d1 [formerly 8f2f5188].


Former-commit-id: 9d67f5b8

Delete all CMT requirements files and other obsolete content in the cmt/
directories. Some non-trivial content of unknown usage has been kept.


Former-commit-id: 972e8215

endreq is deprecated, and replaced with endmsg.

Did not replace in log files, or READMEs. Two files which are questionable are
   InnerDetector/InDetAlignAlgs/InDetAlignGenAlgs/src/GeoModelTRT.dev
and
   InnerDetector/InDetRecTools/InDetMultipleVertexSeedFinderUtils/src/PVFindingTrackFilter_cxx


Former-commit-id: 699dba0d

	* Tagging RecEventAthenaPool-00-00-03.
	* CMakeLists.txt: Fix cmake warning.


Former-commit-id: 2a416a15

	* Tagging RecEventAthenaPool-00-00-02.
	* cmt/requirements: Fix checkreq warning.