[graphite] Consolidate functionality in the SkShaderCodeDictionary

At some point we'll need to go through the dictionary for user provided SkSL

Bug: skia:12701
Change-Id: I484ae30626dad64f2bce1e0948071380d9f8282e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504596
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
22 files changed