lib/versioner: Minor fixes in comments and error message (#9031)

* lib/versioner: Factor out DefaultPath constant.

Replace several instances where .stversions is named literally to all
use the same definition in the versioner package.  Exceptions are the
packages where a cyclic dependency on versioner is impossible, or some
tests which combine the versions base path with other components.

* lib/versioner: Fix comment about trash can in simple versioner.

* lib/versioner: Fix wrong versioning type string in error message.

The error message shows the folder type instead of the versioning
type, although the correct field is used in the comparison.

lib/versioner/util.go

@@ -28,6 +28,10 @@ var (
 	errFileAlreadyExists = errors.New("file already exists")
 )
 
+const (
+	DefaultPath = ".stversions"
+)
+
 // TagFilename inserts ~tag just before the extension of the filename.
 func TagFilename(name, tag string) string {
 	dir, file := filepath.Dir(name), filepath.Base(name)
@@ -258,7 +262,7 @@ func restoreFile(method fs.CopyRangeMethod, src, dst fs.Filesystem, filePath str
 func versionerFsFromFolderCfg(cfg config.FolderConfiguration) (versionsFs fs.Filesystem) {
 	folderFs := cfg.Filesystem(nil)
 	if cfg.Versioning.FSPath == "" {
-		versionsFs = fs.NewFilesystem(folderFs.Type(), filepath.Join(folderFs.URI(), ".stversions"))
+		versionsFs = fs.NewFilesystem(folderFs.Type(), filepath.Join(folderFs.URI(), DefaultPath))
 	} else if cfg.Versioning.FSType == fs.FilesystemTypeBasic && !filepath.IsAbs(cfg.Versioning.FSPath) {
 		// We only know how to deal with relative folders for basic filesystems, as that's the only one we know
 		// how to check if it's absolute or relative.