Previously the command considered non-absolute source file paths relative to
the associated target on the LHS. This causes problems in incremental builds
where files are added from subdirectories and forces users to workaround by
manually converting to absolute paths. Change this to enable more intuitive
usage by projects.
Fixes#17981
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.
