Print warning if an nf-core module no longer exists.

[mahesh-panchal]

Description of feature

Once a module has been installed from nf-core modules, it may be that it needs to be moved from tool to tool/subtool/ to accommodate the addition of further subtools. In this case the normal deprecation guidelines do not apply and the main.nf and corresponding files have to be deleted because of checks on nf-core modules. As a result, if a user who has already installed the module runs an update, nf-core tools will check for the file and not find it there.
The user should get a warning that the file could have been moved, and additionally the option to keep or remove the module locally.