This refactoring is part of a hack which will allow us to add glslang
for use by unit tests and benchmarks, but skip it when PowerVR already
provides glslang.

Bug: b/158655757
Change-Id: I1991ea5ea7524ba71ce9bc2c1d55114318ba434f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45648
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>