Remove "internal" access modifiers from code

[richardtop]

Since "internal" access modifier is the default one, it can be safely removed. Or are there any reasons to actually explicitly state that access modifier for methods/classes?
e.g.: https://github.com/square/Valet/blob/main/Sources/Valet/Internal/Keychain.swift#L20

if this is OK, I can make a PR, or feel free to close the issue if you'd like to keep it this way.

[dfed]

Indeed the modifier is not necessary. Explicit access control was a style I played with back when we rewrote Valet in Swift. Happy to remove it. Feel free to submit a PR.