FetchContent: Restore support for multiple URL values

In c2044fdf3f (FetchContent: Respect the CMP0135 policy setting,
2022-06-02), the URL keyword was wrongly assumed to only have
a single value. Multiple URL values are allowed if they are all
non-local. Rework the logic to remove that incorrect assumption
and handle both single and multi-value URL combinations.

Fixes: #23792