Remove Imported Target Aliasing, master branch (2020.12.15.)
Switched back to deep copying imported targets in ATLAS projects. This ended up being necessary to make partial builds on top of a release/nightly just that much easier to use. Aliases, as I was reminded by @jburr in ATLINFR-3895, just don't do the job satisfactorily.
At least I re-wrote the original code (removed by !780 (merged)) a bit. So that the most "tricky" part of it (where we "deep copy" the imported targets) would be in AtlasCMake, and not in the exported/frozen configuration of the installed ATLAS release. (Making it possible to perform patches on top of the code as part of local compilation.)