Switch to gometalinter

Only small amount of required checks is enabled,
plan is to enable more linters as issues are fixed in the code.

files/package_pool.go

@@ -2,12 +2,13 @@ package files
 
 import (
 	"fmt"
-	"github.com/smira/aptly/aptly"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sync"
+
+	"github.com/smira/aptly/aptly"
 )
 
 // PackagePool is deduplicated storage of package files on filesystem

files/public.go

@@ -2,11 +2,12 @@ package files
 
 import (
 	"fmt"
-	"github.com/smira/aptly/aptly"
 	"io"
 	"os"
 	"path/filepath"
 	"syscall"
+
+	"github.com/smira/aptly/aptly"
 )
 
 // PublishedStorage abstract file system with public dirs (published repos)