- point to main Calamares site in the 'part of' headers instead
of to github (this is the "this file is part of Calamares"
opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
this is the 3-paragraph summary of the GPL-3.0-or-later, which has
a meaning entirely covered by the SPDX tag.
- These don't have to be static methods of PackageItem, a free
function is more convenient.
- Since it's not API of PackageItem anymore, need to
- update tests not to use API
- do API-not-available warnings in consumers
- Put the implementation entirely in a separate file, keep the
not-supported one in PackageModel.cpp (but only in an #ifdef).
- Makes the various optional-data-sources more similar.
