Add a warnig when using source = "."

[thunderhook]

The following issue is about disallowing source = ".":

• Do not allow the use of source="." mapstruct#3631

We might add a warning to discourage users from using it.

Maybe also providing a quick-fix to change it to the parameter name (maybe find a better text):

• Replace source "." with "customer"

When there are multiple parameters provide a quick fix for each parameter, e.g. customer and age:

• Replace source "." with "customer"
• Replace source "." with "age"