Switching to INSTALL_CONFIG=OFF breaks tests by not having them
in the build directory. Some logic was coupling not-installing
to not-using-in-build-dir too closely.
Although the NO_INSTALL keyword could be specified for
Calamares plugins, it didn't actually do anything. Now
it does. A NO_INSTALL module does not install configs
or libraries.
Since we install the CMake files now, also put license headers in them.
These files are intended to help plugin and branding component authors
write them in a simple Calamares-idiomatic way.
