Use Go context to abort gracefully mirror updates

There are two fixes here:

1. Abort package download immediately as ^C is pressed.
2. Import all the already downloaded files into package pool,
so that next time mirror is updated, aptly won't download them
once again.

deb/package.go

@@ -622,6 +622,7 @@ type PackageDownloadTask struct {
 	File         *PackageFile
 	Additional   []PackageDownloadTask
 	TempDownPath string
+	Done         bool
 }
 
 // DownloadList returns list of missing package files for download in format