Improving helpful output from kpm via kcl

[MPV]

The "help" instructions maybe could be clearer? See my journey below for example:

$ kcl mod init

$ kcl mod pkg
run 'kpm pkg help' for more information
the directory where the tar is generated is required

$ kpm pkg help
zsh: command not found: kpm

$ kcl pkg help
Error: source is nil

$ kcl mod pkg help
run 'kpm pkg help' for more information
the directory where the tar is generated is required

$ kcl version
0.11.1-darwin-arm64

😊

[Peefy]

cc @zong-zhe