refactor(server): Replace log calls with logrus

This introduces a structured logging library that can be used (next
step) to attach additional metadata to log entries.

tools/nixery/server/builder/archive.go

@@ -10,11 +10,11 @@ package builder
 import (
 	"archive/tar"
 	"io"
-	"log"
 	"os"
 	"path/filepath"
 
 	"github.com/google/nixery/layers"
+	log "github.com/sirupsen/logrus"
 )
 
 // Create a new tarball from each of the paths in the list and write the tarball

tools/nixery/server/builder/builder.go

@@ -26,7 +26,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"log"
 	"net/http"
 	"net/url"
 	"os"
@@ -38,6 +37,7 @@ import (
 	"github.com/google/nixery/config"
 	"github.com/google/nixery/layers"
 	"github.com/google/nixery/manifest"
+	log "github.com/sirupsen/logrus"
 	"golang.org/x/oauth2/google"
 )
 

tools/nixery/server/builder/cache.go

@@ -19,11 +19,11 @@ import (
 	"encoding/json"
 	"io"
 	"io/ioutil"
-	"log"
 	"os"
 	"sync"
 
 	"github.com/google/nixery/manifest"
+	log "github.com/sirupsen/logrus"
 )
 
 // LocalCache implements the structure used for local caching of