FindThreads: rename imported target to Threads::Threads
Merge topic 'FindThreads_overhaul' into next

36f4c2a3 FindThreads: rename imported target to Threads::Threads
FindThreads: introduce an imported target to link to

This not only holds the library, but can also hold compiler flags needed, e.g.
the -pthread flag preferred by gcc on some platforms. There was no clean way
to get that compiler flag from the module until now.
FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
Merge topic 'FindThreads_overhaul' into next

b7e5c5a2 FindThreads: introduce THREADS_PREFER_PTHREAD_FLAG (#14767)
bcb0e387 FindThreads: introduce an imported target to link to