Follow the pattern of checks that are made for INTERFACE_INCLUDE_DIRECTORIES.
Existence is already checked by cmSourceFile::GetFullPath. Add a check
to disallow relative paths in source directories. Otherwise code such as
target_sources(lib1 INTERFACE foo.cpp)
would fail if consumed by a target in a different directory.
Unlike the INTERFACE_INCLUDE_DIRECTORIES behavior, we don't care whether
the entry comes from an IMPORTED target or not. In the include directories
case, the directory for a non-imported target might not exist yet but
might be created. In the sources case, a file which does not yet
exist in the filesystem must be explicitly marked with the GENERATED
property.
Adjust existing tests and add a new test for the error.
Stephen Kelly