build: use shared logic to build packages (#4202)

* build: use shared logic to build packages

* No longer creates the whole set of gulp tasks just to build a single package.
* Makes the package building & releasing more consistent. No extra `build:release` just for the `lib` package.

References #4108

* Address feedback

scripts/release/publish-build-artifacts.sh

@@ -21,7 +21,7 @@ repoUrl="https://github.com/angular/material2-builds.git"
 repoDir="tmp/$repoName"
 
 # Create a release of the current repository.
-$(npm bin)/gulp build:release
+$(npm bin)/gulp material:build-release:clean
 
 # Prepare cloning the builds repository
 rm -rf $repoDir