upload/koji: use CGInitBuild and clarify metadata structs

Move to requiring CGInitBuild to be called before CGImport. In the
future we could make the former optional again, but for now we want to
allow the caller to have done CGInitBuild and for composer only to do
the CGImport using the passed in build_id and token.

Also rename and document some struct fields in the metadata struct to
make them more specific to our use-case and hopefully easier to read.

Signed-off-by: Tom Gundersen <teg@jklm.no>

internal/target/koji_target.go

@@ -3,6 +3,9 @@ package target
 type KojiTargetOptions struct {
 	BuildID         uint64 `json:"build_id"`
 	Token           string `json:"token"`
+	Name            string `json:"name"`
+	Version         string `json:"version"`
+	Release         string `json:"release"`
 	Filename        string `json:"filename"`
 	UploadDirectory string `json:"upload_directory"`
 	Server          string `json:"server"`