Summary:
Resolve path at callsite rather than in `Config.loadFile`
`Config.loadFile` should not expose unexpected behavior as joining paths together. This moves that responsibility to the call site. `path.resolve` returns the second argument if it is an absolute path.
Reviewed By: bestander
Differential Revision: D4986130
fbshipit-source-id: c80a588ffa86011bcd5a2c393ad5d6eedc6c61ae