Phase Extension Convention

[borkaehw]

Once we start writing more custom phase extensions to the repository, we need to set up a convention to follow. Rules like

• The name of phase .bzl file (e.g. phase_scalafmt.bzl)
• The name of extension .bzl file (e.g. phase_scalafmt_ext.bzl)
• The object names in extension file
  • Extension object (e.g. ext_scalafmt)
  • Rule to provide ScalaRulePhase (e.g. scalafmt_singleton)

Once we have agreements, I am happy to write it to a README.

[ittaiz]

I agree.
Sounds to me like #965 needs to be defined first, no? If so we should advance (hopefully next week) it and then indeed agree on this