In a Android gradle build, how to change the package name by build type?

A bit late on with the answer, but you could have a common part of the applicationID defined such as:

android { 
  defaultConfig { 
    applicationId 'net.company.appname'
    ...
  }
}

and have your flavours specify the applicationIdSuffix as in your example, so that'd give you the 'net.company.appname.debug' and 'net.company.appname.release' or whatever you say you want the suffix to be.