AtlasHepMC Fix, master branch (2021.07.15.)

Made the HepMC library paths relocatable on the interface libraries by replacing the "internal variables" used previously by the package's configuration, with the variables that users are actually supposed to use. (Note that when using HepMC3, the configuration was already using the correct variables.) This is to fix the issue described in ATEAM-768.

While at it, I also simplified the setup of the HEPMC3=1 definitions on the libraries, when building the release against HepMC3. The previous formalism, including the -D prefixes, does work. But CMake's preference is not to include -D in these definitions. Since it has to remove them during the configuration reading anyway, as not every single compiler uses -D for passing definitions/macros to the pre-processor.

Also, I moved the find_package(...) calls inside of the if(...) statements. Not to unnecessarily clutter the configuration log with messages about HepMC(3) not being found.

Tagging @averbyts.