node to page: Refactor the build process

To make it easier to follow and understand.

Both building and rebuilding now follow a four step flow:

1. Init
2. Process
3. Assemble
4. Render

And now there are only one Build method, used for both builds and rebuilds.

Updates #2297

hugolib/hugo_sites_test.go

@@ -543,7 +543,8 @@ func TestMultiSitesRebuild(t *testing.T) {
 		if this.preFunc != nil {
 			this.preFunc(t)
 		}
-		err = sites.Rebuild(cfg, this.events...)
+
+		err = sites.Build(cfg, this.events...)
 
 		if err != nil {
 			t.Fatalf("[%d] Failed to rebuild sites: %s", i, err)