Add namespace to templates

As discussed in the #30272 report, Joomla does not support namespaced templates, this commit solves that.

**Changes**
* the `create()` method now checks for namespaced `template` extension types
* the `getNamespaces()` method properly recognize *Administrator* and *Site* templates; in addition, it will look for `templateDetails.xml` file instead of `extensionName.xml` and read the designated namespace (Line 182)


**Other change**
* replaced `defined('_JEXEC') or die;` with `defined('JPATH_PLATFORM') or die;`, perhaps this change was also expected? Either way, it was my best guess.


**Test setup - also a good piece of documentation draft**
In order to enable namespace for templates, here are the few steps, we'll use the default site template:
* open `site/templates/cassiopeia/templateDetails.xml` and add between `<description>` and `<files>` the following
```markup
<namespace path="src">Joomla\Template\Cassiopeia</namespace>
```
* delete `site/administrator/cache/autoload_psr4.php`
* login to your `site/administrator`
* open the newly created `site/administrator/cache/autoload_psr4.php`, the last element of the return object should be
```
'Joomla\\Template\\Cassiopeia\\Site\\' => [JPATH_SITE . '/templates/cassiopeia/src']
```
* *optional*:  create a custom field in `site/templates/cassiopeia/src/Field` folder and add it to the `templateDetails.xml` along with `addfieldprefix="Joomla\Template\Cassiopeia\Site\Field"` attribute added to the parent `<fieldset>`
* *optional* try and call the newly added custom field from your namespaced template into a module/plugin, test it out


**Questions**
* should I commit this change to the `4.1-dev` branch as well?
* should I commit this change to the `staging` branch? If yes, where is the `namespacemap.php` file?

libraries/namespacemap.php

@@ -6,7 +6,7 @@
  * @license    GNU General Public License version 2 or later; see LICENSE.txt
  */
 
-defined('_JEXEC') or die;
+defined('JPATH_PLATFORM') or die;
 
 use Joomla\CMS\Filesystem\File;
 use Joomla\CMS\Filesystem\Folder;
@@ -35,7 +35,7 @@ class JNamespacePsr4Map
 	 */
 	public function exists()
 	{
-		return is_file($this->file);
+		return file_exists($this->file);
 	}
 
 	/**
@@ -65,6 +65,7 @@ public function create()
 		$extensions = array_merge(
 			$this->getNamespaces('component'),
 			$this->getNamespaces('module'),
+			$this->getNamespaces('template'),
 			$this->getNamespaces('plugin'),
 			$this->getNamespaces('library')
 		);
@@ -139,7 +140,7 @@ protected function writeNamespaceFile($elements)
 	 */
 	private function getNamespaces(string $type): array
 	{
-		if (!in_array($type, ['component', 'module', 'plugin', 'library'], true))
+		if (!in_array($type, ['component', 'module', 'plugin', 'template', 'library'], true))
 		{
 			return [];
 		}
@@ -153,6 +154,10 @@ private function getNamespaces(string $type): array
 		{
 			$directories = [JPATH_SITE . '/modules', JPATH_ADMINISTRATOR . '/modules'];
 		}
+		elseif ($type === 'template')
+		{
+			$directories = [JPATH_SITE . '/templates', JPATH_ADMINISTRATOR . '/templates'];
+		}
 		elseif ($type === 'plugin')
 		{
 			$directories = Folder::folders(JPATH_PLUGINS, '.', false, true);
@@ -172,11 +177,13 @@ private function getNamespaces(string $type): array
 				$extensionPath = $directory . '/' . $extension . '/';
 
 				// Strip the com_ from the extension name for components
+				// $name = $type === 'component' ? str_replace('com_', '', $extension, $count) : $extension; 
 				$name = str_replace('com_', '', $extension, $count);
+				$name = $type === 'template' ? 'templateDetails' : $name; 
 				$file = $extensionPath . $name . '.xml';
 
 				// If there is no manifest file, ignore. If it was a component check if the xml was named with the com_ prefix.
-				if (!is_file($file))
+				if (!file_exists($file))
 				{
 					if (!$count)
 					{
@@ -185,7 +192,7 @@ private function getNamespaces(string $type): array
 
 					$file = $extensionPath . $extension . '.xml';
 
-					if (!is_file($file))
+					if (!file_exists($file))
 					{
 						continue;
 					}