Improve consistency of naming internal functions

[teunbrand]

This PR aims to fix #6024.

Briefly, it enforces some consistency on the various check_*() and validate_*() functions in ggplot2's unexported functions.

Check functions:

• Check input verbatim
• Their purpose is to throw conditions when input is unexpected
• Return nothing (that is, invisible(NULL))

Validator functions:

• Can take different variants of input
• Their purpose is to converge output from input
• Return a single type of output

In addition, a smaller category are functions that take possibly deprecated values and restore it to valid values. I've named these update_*() functions here, but that might be confusing and perhaps we could choose a better name.