Plugins containing "theme" should to be treated as themes

Sommerregen/grav-plugin-themer#6
`./bin/gpm -vvv direct-install https://github.com/Sommerregen/grav-plugin-themer/archive/v1.1.0.zip`

Author: Raphaël Droz

system/src/Grav/Common/GPM/GPM.php

@@ -575,10 +575,10 @@ public static function getPackageType($source)
 
         // either theme or plugin
         $name = basename($source);
-        if (Utils::contains($name, 'theme')) {
+        if (Utils::contains($name, 'theme-')) {
             return 'theme';
         }
-        if (Utils::contains($name, 'plugin')) {
+        if (Utils::contains($name, 'plugin-')) {
             return 'plugin';
         }
         foreach (glob($source . '*.php') as $filename) {