Refactor internal components into `internal` folder

[joshblack]

This is a follow-up to our ADR on Internal Modules. This issues captures the remaining internal components or helpers that we should consider migrating into src/internal to prevent being imported outside of the package.

• _CheckboxOrRadioGroup PR
• _getGlobalFocusStyles PR
• _InputCaption PR
• _InputLabel PR
• _InputValidation PR
• _sharedCheckboxAndRadioStyles PR
• _TextInputInnerAction PR
• _TextInputInnerVisualSlot PR
• _TextInputWrapper PR
• _UnstyledTextInput PR
• _ValidationAnimationContainer PR
• _VisuallyHIdden

[lesliecdubs]

Thanks for filing @joshblack! Should this issue go into our Primer teams backlog "inbox" for triage by the team, or did you have other plans for it?

[joshblack]

Hey @lesliecdubs! No plans in particular, just was something that came up in PR reviews last week. Inbox triage could be a great place for it 👍