The function InstanceBuilder::ProcessImports got long and hard to
understand. This CL is the first of a series which aims at extracting
parts of ProcessImports into specialized functions.

This CL extracts the processing of imported functions into a separate
function. Aside from copying the code, I made the following changes:
* Use {enabled_} instead of a newly created WasmFeatures object.
  - I think this is more correct anyways.
* Rename {index} to {import_index}.
* Load {native_module} from {instance} for every function, instead of
  loading it once for all imports.

R=clemensh@chromium.org

Bug: v8:8562
Change-Id: I9533f302929eedd395962253c340ba35324df631
Reviewed-on: https://chromium-review.googlesource.com/c/1382467
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58331}