Introduces three new types representing each of the possible package
sources and moves the logic for specifying the package source to the
server.
Concrete changes:
* Determining whether a specified git reference is a commit vs. a
branch/tag is now done in the server, and is done more precisely by
using a regular expression.
* Package sources now have a new `CacheKey` function which can be used
to retrieve a key under which a build manifest can be cached *if*
the package source is not a moving target (i.e. a full git commit
hash of either nixpkgs or a private repository).
This function is not yet used.
* Users *must* now specify a package source, Nixery no longer defaults
to anything and will fail to launch if no source is configured.
This test, after performing the usual Nixery build, loads the built
image into Docker, runs it, pulls an image from Nixery and runs that
image.
To make this work, there is some configuration on the Travis side.
Most importantly, the following environment variables have special
values:
* `GOOGLE_KEY`: This is set to a base64-encoded service account key to
be used in the test.
* `GCS_SIGNING_PEM`: This is set to a base64-encoded signing key (in
PEM) that is used for signing URLs.
Both of these are available to all branches in the Nixery repository.
