folly:wangle: fixup libevent detection and export
folly:wangle: fixup libevent detection and export

Summary:
I finally got to the bottom of the weird `-levent` or `event.lib`
linker errors that crop up in the cmake builds.

The issue is that if you have installed a recent libevent it deploys
a cmake CONFIG package that exports just a bare `event` target as the
dep on libevent.  This is unfortunate because it is interpreted as
meaning `-levent` with no library path, which for an installation
of libevent that is not in the default path will result in linker
errors in libraries downstream of folly.

To resolve this, I've given the common `FindLibEvent.cmake` file
(that was derived from the folly library of the same name) a similar
treatment to the recent changes to find glog and taught it how to
locate libevent from a config package and to fall back on a basic
include/library check.

If we find an `event` target then we extract the actual library
path from it and export that.

I've removed folly's and wangle's own FindLibEvent.cmake so that it will pick
up the common library which gets shipit sync'd out to
https://github.com/facebook/folly/blob/master/CMake/FindLibEvent.cmake
and https://github.com/facebook/wangle/blob/master/build/fbcode_builder/CMake/FindLibEvent.cmake

Reviewed By: yfeldblum

Differential Revision: D14702577

fbshipit-source-id: d35d9f741e009dcd77976c0637ba3024a8a4aef3

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