internal/buildcfg: refactor GOEXPERIMENT parsing code somewhat

This CL extracts out a ParseGOEXPERIMENT helper function that parses
GOOS/GOARCH/GOEXPERIMENT values and returns active and baseline
experiment flag sets and an error value, without affecting any global
state. This will be used in the subsequent CL for 'go env' support for
GOEXPERIMENT to validate configuration changes.

The existing package initialization for Experiment and
experimentBaseline and also UpdateExperiments are updated to use it as
well.

Change-Id: Ic2ed3fd36d2a6f7f3d8172fccb865e02505c0052
Reviewed-on: https://go-review.googlesource.com/c/go/+/331109


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>