appc: fix relative paths in app validator

strings.TrimPrefix was not correct since it resulted in different behaviour
depending on whether the directory passed had a trailing slash or not (this
meant for example that `actool validate foo/` would succeed but `actool
validate foo` would fail) Go has a function for the correct behaviour in
filepath.Rel, so use it.

Also small style updates to keep up with the recent 
appmanifest+filesetmanifest --> ImageManifest change