This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.
This is the first step towards factoring out CLI-based functionality
for Nixery.
This case should not be possible unless something manually constructs
a logrus entry with a non-error value in the log.ErrorKey field, but
it's better to be safe than sorry.
Uses a hash of Nixery's sources as the version displayed when Nixery
launches or logs an error. This makes it possible to distinguish
between errors logged from different versions.
The source hashes should be reproducible between different checkouts
of the same source tree.
This formatter has basic support for the Stackdriver Error Reporting
format, but several things are still lacking:
* the service version (preferably git commit?) needs to be included in
the server somehow
* log streams should be split between stdout/stderr as that is how
AppEngine (and several other GCP services?) seemingly differentiate
between info/error logs
