Fix discovery of `libsodium`
Fix discovery of `libsodium`

Summary:
By putting this in `fizz-config.cmake`, we can use depend on the `sodium` target without compromising our dependents ability to find the library.

Put the search module in the common location under `fbcode_builder/CMake` to let dependents use it.

Reviewed By: yfeldblum

Differential Revision: D24686041

fbshipit-source-id: 942d1ab34feef6cadac2b584eb8cb2d999bae0ca

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly