Fix registration of yo sub-generators in setup-workspace and build.js

[oltionchampari]

Closes #263

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Summary of changes

• The problem was that we were using env.register() to register a subgenerator

• That doesn't work when other subgenerators are called within it .

• Replaced env.register(require.resolve('../' + generator), 'phovea:' + generator);

  with

   env.lookup(() => {
         env.run(`phovea:${generator} ${_args}`, options || {}, () => {
           // wait a second after running yo to commit the files correctly
           setTimeout(() => resolve(), 500);
         });
        });
  

env.lookup scans for all installed generator/subgenerators.

As stated here: https://stackoverflow.com/questions/48680805/run-a-yeoman-sub-generator-using-yeoman-environment

• I tested the command setup-workspace in generator-phovea v 1.1.1 and v 2.0.1 and they create
  they create the same files .
• Further testing though is needed though.

[thinkh]

Works well! 👍