Add new target-property `IMPORTED_GLOBAL`.
The purpose of this new `IMPORTED_GLOBAL` target-property is to prolong the lifetime and scope of `IMPORTED` targets in such a way as if they had been created with the keyword `GLOBAL` in the first place. * It can only be set to `TRUE`. That means, a local `IMPORTED` target can be promoted to global scope but a global `IMPORTED` target cannot be degraded to local scope! * Setting it to `TRUE` only succeeds if done from within the same directory in which the `IMPORTED` target was created in the first place. Fixes #17256.
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