Remove array keys in package repositories. Fixes #109

When merging repositories in the MultiRepository, a theme and plugin with the
same slug would create a conflict. The keys aren't exposed outside of the
repository objects and weren't used internally.

src/Repository/FilteredRepository.php

@@ -58,7 +58,7 @@ public function all(): array {
 
 		foreach ( $this->repository->all() as $package ) {
 			if ( ( $this->callback )( $package ) ) {
-				$packages[ $package->get_slug() ] = $package;
+				$packages[] = $package;
 			}
 		}
 

src/Repository/InstalledPlugins.php

@@ -52,7 +52,7 @@ public function all(): array {
 		foreach ( get_plugins() as $plugin_file => $plugin_data ) {
 			$package        = $this->build( $plugin_file, $plugin_data );
 			$slug           = $package->get_slug();
-			$items[ $slug ] = $package;
+			$items[] = $package;
 		}
 
 		ksort( $items );

src/Repository/InstalledThemes.php

@@ -51,7 +51,7 @@ public function all(): array {
 		$items = [];
 
 		foreach ( wp_get_themes() as $slug => $theme ) {
-			$items[ $slug ] = $this->build( $slug, $theme );
+			$items[] = $this->build( $slug, $theme );
 		}
 
 		return $items;