refactor(server): Move package source management logic to server

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.

tools/nixery/server/builder/builder.go

@@ -118,15 +118,16 @@ func BuildImage(ctx *context.Context, cfg *config.Config, cache *BuildCache, ima
 			return nil, err
 		}
 
+		srcType, srcArgs := cfg.Pkgs.Render(image.Tag)
+
 		args := []string{
 			"--timeout", cfg.Timeout,
 			"--argstr", "name", image.Name,
 			"--argstr", "packages", string(packages),
+			"--argstr", "srcType", srcType,
+			"--argstr", "srcArgs", srcArgs,
 		}
 
-		if cfg.Pkgs != nil {
-			args = append(args, "--argstr", "pkgSource", cfg.Pkgs.Render(image.Tag))
-		}
 		cmd := exec.Command("nixery-build-image", args...)
 
 		outpipe, err := cmd.StdoutPipe()