[build] do not create non-default new opam switches as aliases
[build] do not create non-default new opam switches as aliases

Summary:
For instance, `./build-infer.sh --opam-switch "4.04.2+spacetime"` now builds an
actual spacetime switch instead of a clone of the 4.04.2 switch called
"4.04.2+spacetime".

Reviewed By: jberdine

Differential Revision: D5631258

fbshipit-source-id: e6295c5

ropas/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,ropas/infer,ropas/infer,facebook/infer,facebook/infer,blzq/infer,ropas/infer,facebook/infer,ropas/infer,blzq/infer,jvillard/infer,blzq/infer,jvillard/infer,ropas/infer,jvillard/infer,blzq/infer,blzq/infer,facebook/infer,blzq/infer