Eliminate simple systems commands

[michalfita]

Problem

In multiple places the plugin unnecessary calls commands for file operations, what's bad practice and non-portable solution. Example:

out, err = exec.Command("cp", "-rf", archivePath, dst).CombinedOutput()

Proposal

Replace use of external invocation of standard Unix commands with standard library equivalents of file operations or (when they're missing from API¹) with Go packages.

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¹ lack of file copying makes me coming from Rust 🤣