This clarifies the difference between "collect" and "pack" methods.
Previously the naming was overlapping between the two. Also makes
the "packUserVaryings" method private.

Refactoring change only. Will enable further improvements for
capturing varyings with multiple shader stages.

Bug: angleproject:5496
Change-Id: I854590ceab39f3a0e7a785516e0d1fd44e4ccc98
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606529
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>