LCG Executables, master branch (2020.04.24.)

This MR was broken off of !664 (closed), to reduce the size of that MR a bit.

Here I introduce a mechanism for setting up <PACKAGE>_<executable>_EXECUTABLE variables in all of the find-modules that set up some. Previously these modules would only make sure that the paths with the executables would be added to the runtime environment of the project.

I also had to make a small change in PyModules and flake8_atlas to account for the ${PIP_EXECUTABLE} -> ${PIP_pip_EXECUTABLE} name change.

Pinging @fwinkl.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST-CC7 #242]

merged

mentioned in commit 68de176b