fix: avoid wrong warning of explicit public paths

[meteorlxy]

Description

This issue is caused by #4631, reported here https://github.com/vitejs/vite/pull/4631/files#r707910410.

We should not simply determine publicPath by slicing the string, because the custom public dir can be outside the root dir.

It's OK to use startsWith to check if publicDir is inside root or not, because they are all resolved as absoluted path. An alternative way is to use path.relative().

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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