external/CMakeLists.txt: fix find_pkg when unordered_dense installed (#647)
external/CMakeLists.txt: fix find_pkg when unordered_dense installed (#647)

If the user has already installed unordered_dense beforehand,
avoid downloading and installing again.
Applicable when the distribution has packaged unordered_dense.
MikePopoloski/slang,MikePopoloski/slang