Move helpers from datadog-agent to rshell

[julesmcrt]

Some helpers where added into this datadog-agent in this PR DataDog/datadog-agent#49825

1. warnUnnamespacedCommands
2. onlyRshellPrefixedCommands
3. reducePathListToBroadest
4. etc

They would be better in rshell instead.
See DataDog/datadog-agent#49825 (comment)