chore: inline file traversal function

[hyperz111]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
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• Refactoring (no functional changes, no api changes)
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What is the current behavior? (You can also link to an open issue here)

N/A

What is the new behavior?

N/A

Does this PR introduce a breaking change?

• Yes
• No

Other information:

We currently switch from find-up to empathic (using empathic/find entry), we use these for find the package root directory. But find-up and empathic/find has a major (i think) different:

• find-up (in locate-path dependency) is using lstatSync, which is slower but safer. Because we can ensure if the entry is file or not.
• empathic is using existsSync, which is faster but unsafe and can show a new bug. Because existsSync just check if the entry is available, no matter if the entry is a file or directory.

So if we want a safer but don't want to back to find-up, i prefer to inline the function instead. This will reduce the abstraction too i think.