The `extensions` map of ID -> extension has been renamed to `extensionsByID`.
The `extensionsImported` is a new set containing the list of extension names that have been imported. This set can be efficiently queried to determine whether a particular extension is in use.

Bug: b/145351270
Change-Id: I74bbfb2e5b95862c5381037f259262a9d45b6624
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40092Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>