Performance improvements for "build-plugin" task

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

desktop/pkg-lib/src/index.ts

@@ -9,3 +9,4 @@
 
 export {default as runBuild} from './runBuild';
 export {default as getWatchFolders} from './getWatchFolders';
+export {default as computePackageChecksum} from './computePackageChecksum';