Upgrade cake to version 2.0.0

[CharliePoole]

Fixes #1059

[mikkelbu]

We should also update azure-pipelines.yml to use the new format for the pipeline to pass.

BUILDING.md also need updating (and perhaps more places, but I'm on my phone, so searching is not optimal. I can make a better search tomorrow)

[CharliePoole]

Looks like I may have broken something by changing Package so it depends on Build. The Windows job on Azure first builds and tests, then publishes test results and finally re-runs the script to do packaging. I may need to add a "PackageOnly" target.

[CharliePoole]

The Package target continues to depend on Build, to prevent accidentally building the wrong stuff, particularly when used locally. To avoid a build, you have to use the PackageExistingBuild target.