fix($shared-utils): fail to resolve markdown-it plugins (close #2286)(#2289)

Author: meteorlxy

packages/@vuepress/shared-utils/src/moduleResolver.ts

@@ -88,16 +88,15 @@ class ModuleResolver {
     this.allowedTypes = allowedTypes
     this.load = load
     this.cwd = cwd || process.cwd()
+    this.typePrefixLength = type.length + 1
     if (org) {
       this.nonScopePrefix = `${org}-${type}-`
       this.scopePrefix = `@${org}/${type}-`
+      this.prefixSlicePosition = this.typePrefixLength + org.length + 1
     } else {
       this.nonScopePrefix = `${type}-`
+      this.prefixSlicePosition = this.typePrefixLength
     }
-    this.typePrefixLength = type.length + 1
-    /* - */
-    this.prefixSlicePosition = this.typePrefixLength + org.length + 1
-    /* @ */
   }
 
   /**