refactor(server): Convert existing log entries to structured format

This rewrites all existing log statements into the structured logrus
format. For consistency, all errors are always logged separately from
the primary message in a field called `error`.

Only the "info", "error" and "warn" severities are used.

tools/nixery/server/layers/grouping.go

@@ -172,8 +172,14 @@ func (c *closure) ID() int64 {
 
 var nixRegexp = regexp.MustCompile(`^/nix/store/[a-z0-9]+-`)
 
+// PackageFromPath returns the name of a Nix package based on its
+// output store path.
+func PackageFromPath(path string) string {
+	return nixRegexp.ReplaceAllString(path, "")
+}
+
 func (c *closure) DOTID() string {
-	return nixRegexp.ReplaceAllString(c.Path, "")
+	return PackageFromPath(c.Path)
 }
 
 // bigOrPopular checks whether this closure should be considered for
@@ -321,7 +327,10 @@ func dominate(budget int, graph *simple.DirectedGraph) []Layer {
 	})
 
 	if len(layers) > budget {
-		log.Printf("Ideal image has %v layers, but budget is %v\n", len(layers), budget)
+		log.WithFields(log.Fields{
+			"layers": len(layers),
+			"budget": budget,
+		}).Info("ideal image exceeds layer budget")
 	}
 
 	for len(layers) > budget {