Summary:
Few improvements for "build-plugin" task which together with Sandcastle command changes (D26872427) helps to build all plugins in CI ~30% faster if most of them has not changed (which is usually the case):
1) compute package checksum in the same script to not call additional yarn scripts for each plugin
2) avoid packaging plugin if it's checksum has not changed since last release
Reviewed By: mweststrate
Differential Revision: D26872253
fbshipit-source-id: 968102d32a1550ea7503f1169f0ef2863296383f
baeb8ba5be