refactor(factory): improve fallback multiple v-if

[KSH-code]

close #11856

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[KSH-code]

This is the very first time I create Pull Request to vuejs, If description is bad, please let me know. then I'll edit.

[sirlancelot]

You will probably want to make this change to the source file instead of just the weex output: src/core/vdom/helpers/normalize-scoped-slots.js

[KSH-code]

Nice to point out mistake!
files of packages directory are made by build, right?

[KSH-code]

@posva
When it will be merged? it was reviewed over 1 year ago.

[KSH-code]

@posva @yyx990803
When it will be merged? 🤔
I've seen release 2.7 was released.

[KSH-code]

@posva @yyx990803