Feat: add a warning when an imported file is not accessed during container builds

[mikemccracken]

Is your feature request related to a problem? Please describe.

if imports are not used, they should be removed from the stacker file

it'd be nice if stacker warned you that an import was unused

Describe the solution you'd like

It's Hard to look at a script statically and determine that it won't use a given path, but stacker could watch the file during the script run and inform you afterward if it was accessed, using inotify watches

Describe alternatives you've considered

No response

Additional context

No response