fix: change merge webpack priority rawconfig

[liaohainan]

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

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

• Yes
• No

Other information:
when I will use command:
vue-cli-service build --target lib --name myLib [entry]

entry is *.vue

and vue.config.js has webpack config：

config.output.set('library', 'otherLib')

now: the webpack output library will priority myLib

expected result：Command priority webpack config library