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/config/pkgsource.go

@@ -132,21 +132,30 @@ func (p *PkgsPath) CacheKey(pkgs []string, tag string) string {
 // specified, the Nix code will default to a recent NixOS channel.
 func pkgSourceFromEnv() (PkgSource, error) {
 	if channel := os.Getenv("NIXERY_CHANNEL"); channel != "" {
-		log.Printf("Using Nix package set from Nix channel %q\n", channel)
+		log.WithFields(log.Fields{
+			"channel": channel,
+		}).Info("using Nix package set from Nix channel or commit")
+
 		return &NixChannel{
 			channel: channel,
 		}, nil
 	}
 
 	if git := os.Getenv("NIXERY_PKGS_REPO"); git != "" {
-		log.Printf("Using Nix package set from git repository at %q\n", git)
+		log.WithFields(log.Fields{
+			"repo": git,
+		}).Info("using NIx package set from git repository")
+
 		return &GitSource{
 			repository: git,
 		}, nil
 	}
 
 	if path := os.Getenv("NIXERY_PKGS_PATH"); path != "" {
-		log.Printf("Using Nix package set from path %q\n", path)
+		log.WithFields(log.Fields{
+			"path": path,
+		}).Info("using Nix package set at local path")
+
 		return &PkgsPath{
 			path: path,
 		}, nil