Using the official libevent export for findfind_package()

The current `find_package` call looks like:
https://github.com/facebook/folly/blob/8cde5e001a9de1feea442b32444d15985e8167c4/CMake/folly-deps.cmake#L67

However libevent install looks like:
https://github.com/libevent/libevent/blob/e3e7bb212ea17aa8a9d5a30163487342e6ebb350/CMakeLists.txt#L1561-L1563

It would be nice if this was changed to:

```diff
-find_package(LibEvent MODULE REQUIRED)
+find_package(Libevent MODULE REQUIRED)
```

for better integration with other tools and projects