copy,refactor: split copyOneImage and copyMultipleImages

[flouthoc]

As discussed in #1875 it is suggested to keep the two implementation in different files since there independent complexity is increasing.

[flouthoc]

@mtrmac PTAL, the only caller of copyOneImage and copyMultipleImage is func Image( which i kept intact but we can also move parts of it to single and multiple.

[mtrmac]

Thanks!

Please also move

• Smaller utility functions only used in the moved functions (e.g. compareImageDestinationManifestEqual). In particular all of the layer code that still is in copy.go should move to single.go
• Similarly, the global constants/variables that only are used in the moved functions (e.g. compressionBufferSize), to the same files
• imageCopier itself, to single.go (and probably all methods on imageCopier).
• Unit tests should move with the tested functions.

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(If you’d like, I can prepare a PR for these moves — the choice basically depends on how it would be more convenient for you to rebase/update the other outstanding work.)

[flouthoc]

(If you’d like, I can prepare a PR for these moves — the choice basically depends on how it would be more convenient for you to rebase/update the other outstanding work.)

@mtrmac Sure sounds good to me, I'll rebase on top of these changes.

[mtrmac]

(If you’d like, I can prepare a PR for these moves

#1878

[mtrmac]

#1878 was merged.