Use private instead of fileprivate if possible

[JakeLin]

Following up @tbaranes 's comment on #437 (comment)

We should consider using private if possible instead of fileprivate.

Personally, I don't like the idea of having two privates: private and fileprivate. But since we have them, we should follow Principle of least privilege. For example, we should use private for variables and methods if they get used within the same scope (same extension, same class or struct). If we can't, then use fileprivate when those variables and methods get used in different scope but within the same file.

How to do it?

Search all fileprivate and see which one can be downgraded to private.

[phimage]

for the future please look at
https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md
and the rejected one https://github.com/apple/swift-evolution/blob/master/proposals/0159-fix-private-access-levels.md