Cleanup duplication in handle_include_easyblocks_from

[Flamefire]

Seen the linter warning that included_easyblocks may be used before assignment in the check_included_multiple function.
So ensure included_easyblocks is defined before the function.

Also the whole logging and checking code is duplicated in each branch (for the CLI options),
so move the logging from each option branch to a common function.
This can use the function that checks for duplicates as both are just logging the inclusion (the warning is also just a "log")
This makes the whole handling uniform:

• Extract filenames
• Report multiple inclusions
• Report inclusion