* caddycmd: Add `--skip-cleanup` to upgrade commands
This is a partial fix for https://github.com/caddyserver/caddy/issues/4057, making it possible to retain the old build of Caddy, in case something went wrong.
* caddycmd: Fix duplicate error message
The error message "download succeeded, but unable to execute" was repeated, because it was both in the `listModules`/`showVersion` functions and in the calling `upgradeBuild` function. Oversight when this was refactored.
* caddycmd: Implement fix for performing cleanup on Windows
Without this, the cleanup operation would fail with an error message like this:
upgrade: download succeeded, but unable to clean up backup binary: remove C:\caddy\caddy.exe.tmp: Access is denied.
* caddycmd: Rename to `--keep-backup`, simplify build constraints
* adding package command
* add-package command name
* refactoring duplicate code
* fixed by review
* fixed by review
* remove-package command
* commands in different files, common utils
* fix add, remove, upgrade packages in 1 file
* copyright and downloadPath moved
* refactor
* downloadPath do no export
* adding/removing multiple packages
* addPackages/removePackages, comments, command-desc
* add-package, process case len(args) == 0
Co-authored-by: Francis Lavoie <lavofr@gmail.com>
