The additional commands (mounter, gcp-credentials-provider) are needed for E2E
node testing. This change makes e2e_node.test entirely self-contained.
Copying the commands' code into separate packages is temporary and only done to
avoid touching them while it is still unclear whether this approach will work
out.
Besides avoiding changes to the build rules, bundling the functionality also has a
slight size advantage: the size of e2e_node.test increases by 10KB, whereas
the other two separate commands would add 10MB.
