Implementation of layered LCG software stacks

This MR will contain all the changes needed for the implementation of the LCG layered software stacks. In particular it includes:

• Ensure single version of each package with a 'last one wins' policy (SPI-1402). A warning message is reported each time a package gets overwritten. The dev-generators toolchain has been slightly modified to overcome some inconsistencies with this new policy. More work is probably needed.
• Ensure that any LCG toolchain can be included without side effects (SPI-1403). Including heptools-common.cmake and calling LCG_prepare_paths() has been moved out of all toolchains. Existing toolchains have been adapted.
• Introduced new macro LCG_top_packages(...) to define the top level packages (SPI-1404). The target top_packages is defined to build all of them. This is the default with the command lcgcmake install.
• Added new sub-command lcgcmake show packages to list alphabetically sorted list of packages and versions defined after the configuration.
• Cleanup all LCGCMT deprecated logic.
• Removed obsoleted toolchains
• Implemented alternate package source repositories (GIT) (SPI-1406). Updated the toolchain files to make use of this new functionality.