This type will be used for representing possible states of a change
review across different code review systems (GitHub PRs, Gerrit CLs,
and other custom changes).

The possible values are largely based on the code review conventions
set up for the Go project. They are described at
https://groups.google.com/d/msg/golang-dev/YU56eKYiXJg/K1CkXo7Iy0gJ.

This is a starting point, the values may change if compelling reasons
to do so are discovered.

At this time, the go.mod file specifies only the module path,
and nothing more.

In the future, it can be expanded to specify additional information,
such as the required modules and their versions, and the expected
Go language version. This will be considered after some time passes,
and the long-term implications of doing so are better understood.