chore: Refactor OverlayCreate

[dtrudg]

Description of the Pull Request (PR):

Refactor the OverlayCreate routines ahead of work to add overlay creation for OCI-SIF.

• Don't duplicate utility functions from the image package.
• Split up command checks / image checks / ext overlay file create / addition to SIF into separate functions.

Should allow OCI-SIF overlay creation to be added in a neater commit.

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