Change custom component template suffix from .template to .tpl (#244)

Extract the template suffix into a central `TemplateSuffix` constant in
`pkg/registry` and switch the default from `.template` to `.tpl`

Author: LucaBernstein

docs/usage/custom-ocm-components.md

@@ -9,7 +9,7 @@ But if your component is backed by an OCM component descriptor, you can benefit
 
 1. The component directory contains a file named `component-name` containing the OCM component name (e.g. `my.private-github.com/gardener/my-gardener-extension`).
 2. The custom component must be included in the OCM descriptor tree. This means it must be referenced by the root component descriptor or any component referenced directly or indirectly.
-3. The component directory contains template files with suffix `.template` (e.g. `my-extension-deployment.yaml.template`). It can reference values for resources or image vector overrides as extracted from the OCM component descriptor.
+3. The component directory contains template files with suffix `.tpl` (e.g. `my-extension-deployment.yaml.tpl`). It can reference values for resources or image vector overrides as extracted from the OCM component descriptor.
 
 ### How it works
 
@@ -43,7 +43,7 @@ ocm:
 ```
 components/my-gardener-extension
   ├── component-name
-  ├── my-extension-deployment.yaml.template
+  ├── my-extension-deployment.yaml.tpl
   ├── kustomization.yaml
   ...
 ```
@@ -60,7 +60,7 @@ glk resolve ocm -c landscapekitconfiguration.yaml --target-dir /path/to/landscap
 
 4. Make use of the extracted resource information stored in the file `ocm-components.yaml`.
 
-If your component is a gardener extension, the `my-extension-deployment.yaml.template` may look like this:
+If your component is a gardener extension, the `my-extension-deployment.yaml.tpl` may look like this:
 ```yaml
 apiVersion: operator.gardener.cloud/v1alpha1
 kind: Extension

pkg/registry/types.go

@@ -19,6 +19,8 @@ import (
 const (
 	// CustomComponentNameFilename is the filename containing a custom OCM component name.
 	CustomComponentNameFilename = "component-name"
+	// TemplateSuffix is the file extension used to identify custom component template files.
+	TemplateSuffix = ".tpl"
 )
 
 // Interface is the interface for a component registry.
@@ -92,7 +94,7 @@ func (r *registry) renderCustomComponents(ocmComponentName, componentDir string,
 		if err != nil {
 			return err
 		}
-		if info.IsDir() || !strings.HasSuffix(info.Name(), ".template") {
+		if info.IsDir() || !strings.HasSuffix(info.Name(), TemplateSuffix) {
 			return nil
 		}
 		content, err := opts.GetFilesystem().ReadFile(path)
@@ -107,7 +109,7 @@ func (r *registry) renderCustomComponents(ocmComponentName, componentDir string,
 		if err != nil {
 			return fmt.Errorf("error rendering template file %s for custom component %s: %w", path, ocmComponentName, err)
 		}
-		targetFile := strings.TrimSuffix(path, ".template")
+		targetFile := strings.TrimSuffix(path, TemplateSuffix)
 		if err := opts.GetFilesystem().WriteFile(targetFile, renderedContent, 0600); err != nil {
 			return fmt.Errorf("error writing rendered template file %s for custom component %s: %w", targetFile, ocmComponentName, err)
 		}