Minor refactoring (#262)

- Move frontmatter-related operation from Product to ProductFrontmatter. This makes more senses, as we are manipulating different files / kind of data.
- Use Product directly to load old versions.

src/git.py

@@ -9,9 +9,11 @@ METHOD = 'git'
 
 p_filter = sys.argv[1] if len(sys.argv) > 1 else None
 for product_name, configs in endoflife.list_products(METHOD, p_filter).items():
-    print(f"::group::{product_name}")
-    product = endoflife.Product(product_name, load_product_data=True)
-    for config in product.get_auto_configs(METHOD):
+    product = endoflife.Product(product_name)
+    print(f"::group::{product.name}")
+
+    product_frontmatter = endoflife.ProductFrontmatter(product.name)
+    for config in product_frontmatter.get_auto_configs(METHOD):
         git = Git(config.url)
         git.setup(bare=True)