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.

Tests/ConfigSources/CMakeLists.txt

@@ -5,9 +5,9 @@ project(ConfigSources)
 
 add_library(iface INTERFACE)
 set_property(TARGET iface PROPERTY INTERFACE_SOURCES
-  iface_src.cpp
-  $<$<CONFIG:Debug>:iface_debug_src.cpp>
-  $<$<CONFIG:Release>:does_not_exist.cpp>
+  "${CMAKE_CURRENT_SOURCE_DIR}/iface_src.cpp"
+  "$<$<CONFIG:Debug>:${CMAKE_CURRENT_SOURCE_DIR}/iface_debug_src.cpp>"
+  "$<$<CONFIG:Release>:${CMAKE_CURRENT_SOURCE_DIR}/does_not_exist.cpp>"
 )
 
 add_executable(ConfigSources