refactor: rsync and scp into copytool pkg

[unsuman]

Yes, I think we should make a separate package rsync same as https://github.com/AkihiroSuda/alcless/blob/master/pkg/rsync/rsync.go
but this can be done by me in another PR?

Originally posted by @unsuman in #4429 (comment)

Depends on #4429

[unsuman]

Left with the refactoring of rsync in the cmd/limactl/shell.go file with these current changes.
Since there exists bats testing for the copy and shell --sync command, is there a need to add unit tests for this new copytool package?

[AkihiroSuda]

Needs rebase