build system updates
build system updates

Summary:
* remove folly automagic, leave installing folly to the user for now
* add FindFolly.cmake
* move CMakeLists.txt to public_tld
* add install commands

Test Plan: built on VM

FB internal diff: D2107275

orian/wangle,mzhaom/wangle,mzhaom/wangle,orian/wangle,facebook/wangle,rklabs/wangle,AresTao/wangle,jamperry/wangle,Orvid/wangle,Orvid/wangle,Dalzhim/wangle,raphaelamorim/wangle,facebook/wangle,raphaelamorim/wangle,rklabs/wangle,AresTao/wangle,jamperry/wangle,facebook/wangle,Dalzhim/wangle