cmTarget: Don't allow relative paths in INTERFACE_SOURCES
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.
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