Add HEAD support for rpm repo files (#28309)

Fixes https://codeberg.org/forgejo/forgejo/issues/1810

zypper uses HEAD requests to check file existence.

https://github.com/openSUSE/libzypp/blob/HEAD/zypp/RepoManager.cc#L2549

https://github.com/openSUSE/libzypp/blob/HEAD/zypp-curl/ng/network/private/downloaderstates/basicdownloader_p.cc#L116

@ExplodingDragon fyi

routers/api/packages/api.go

@@ -520,7 +520,10 @@ func CommonRoutes() *web.Route {
 				r.Get("", rpm.DownloadPackageFile)
 				r.Delete("", reqPackageAccess(perm.AccessModeWrite), rpm.DeletePackageFile)
 			})
-			r.Get("/repodata/{filename}", rpm.GetRepositoryFile)
+			r.Group("/repodata/{filename}", func() {
+				r.Head("", rpm.CheckRepositoryFileExistence)
+				r.Get("", rpm.GetRepositoryFile)
+			})
 		}, reqPackageAccess(perm.AccessModeRead))
 		r.Group("/rubygems", func() {
 			r.Get("/specs.4.8.gz", rubygems.EnumeratePackages)