Fix character group for package target

No need to exclude `[` or `]`.

Author: rafaelfranca

lib/importmap/npm.rb

@@ -147,7 +147,7 @@ def vendored_packages_without_version(packages_with_versions)
     end
 
     def find_unversioned_vendored_package(line, versioned_packages)
-      regexp = line.include?("to:")? /#{PIN_REGEX}to: ["']([^["']]*)["'].*/ : PIN_REGEX
+      regexp = line.include?("to:")? /#{PIN_REGEX}to: ["']([^"']*)["'].*/ : PIN_REGEX
       match = line.match(regexp)
 
       return unless match