AthenaP1 Project, master branch (2021.02.19.)
In order to address the issues discussed in ATLINFR-3977, and then agreed upon in today's GRC meeting, I added a configuration for building the AthenaP1 project from the master branch.
The package composition, at this point, allows the project to build. But I have not made any runtime tests yet. The package filters will most definitely need to be tweaked going forward. (I created the package filter as a mix of https://gitlab.cern.ch/atlas/athena/-/blob/release/21.1.56/Projects/AthenaP1/package_filters.txt and Projects/Athena/package_filters.txt, and tweaked it until the build succeeded.)
Also note that I'm not modifying the Athena project in this MR. Once the AthenaP1 nightlies are in a good shape, and everybody is happy with them, we will need to remember to remove the build of the HLT packages from Athena. (And stop it depending on TDAQ.) But for a transition period we need to keep Athena as is, to provide a reference to compare AthenaP1 against...
I'm making it a WIP for now, since the CI is blind to these changes anyway. So let's not waste its resources while experts look at the MR.