lib/versioner: Reduce surface area (#6186)

* lib/versioner: Reduce surface area

This is a refactor while I was anyway rooting around in the versioner.
Instead of exporting every possible implementation and the factory and
letting the caller do whatever, this now encapsulates all that and
exposes a New() that takes a config.VersioningConfiguration.

Given that and that we don't know (from the outside) how a versioner
works or what state it keeps, we now just construct it once per folder
and keep it around. Previously it was recreated for each restore
request.

* unparam

* wip

lib/versioner/external_test.go

@@ -29,7 +29,7 @@ func TestExternalNoCommand(t *testing.T) {
 
 	// The versioner should fail due to missing command.
 
-	e := External{
+	e := external{
 		filesystem: fs.NewFilesystem(fs.FilesystemTypeBasic, "."),
 		command:    "nonexistent command",
 	}
@@ -62,7 +62,7 @@ func TestExternal(t *testing.T) {
 
 	// The versioner should run successfully.
 
-	e := External{
+	e := external{
 		filesystem: fs.NewFilesystem(fs.FilesystemTypeBasic, "."),
 		command:    cmd,
 	}