Rename 'registry_tool' to 'nomulus'

This changes everything with external visibility beyond the codebase
(i.e. the name of the compiled binary and the documentation that refers
to it). It does not change a lot of things internal to the codebase,
i.e. the "RegistryTool" class didn't change its name. We can rename that
in a subsequent CL if we want to.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135022087

java/google/registry/tools/RegistryCli.java

@@ -85,7 +85,7 @@ final class RegistryCli {
       } else {
         jcommander.usage(jcommander.getParsedCommand());
       }
-      // Don't rethrow if we said: registry_tool command --help
+      // Don't rethrow if we said: nomulus command --help
       if ("Unknown option: --help".equals(e.getMessage())) {
         return;
       }