Update architecture to adapt package structure

[ernilambar]

Continued from #165

Since above PR was branched way earlier, there has been lots of fixes and updates in the repo. Merge conflicts were due to those changes. So I have created separate PR with implementation of changes from the above PR. Also few new changes are done to pass the tests.

This PR makes changes to the package structure to fit the one from the bundled plugins:

• Use WP_CLI\Doctor as the namespace root
• Use PSR-4 autoloader
• Remove unused files
• Update feature test to accommodate namespace changes

[swissspidy]

Amazing!