Put the convertPlainUrlInLink utility function inside the drush_help service

Problem/Motivation

A discussion is open in When I should create a service or an utility function? and it seems that the correct way is have the functions that access to a service inside a service.

Proposed resolution

Put the convertPlainUrlInLink() utility function inside the drush_help service.

Remaining tasks

None.

User interface changes

None.

API changes

Now the convertPlainUrlInLink() method is available in the drush_help service. And the convertPlainUrlInLink() utility function will not longer exists.

Data model changes

None.