Re-structure project layout

[shifucun]

Current project has a flat layout that has Golang packages, cloud build files, bash scripts on the top level folder. At this point, it becomes hard to add deployment framework like Skaffold. Having a better structure would ease the code maintain burden as well.

Here propose to use a standard Golang layout for the mixer repo. The repo structure would look like the following:
.
+-- build/
+-- cmd/
+-- config/
+-- deploy/
+-- docs/
+-- esp/
+-- examples/
+-- gke/
+-- internal/
| +-- base/
| +-- proto/
| +-- sparql/
| +-- translator/
| +-- util/
+-- proto/
+-- scripts/
+-- test/