The source package is provided via --source(-pkg)=.... It can also be
provided by a command line argument starting with a lower case letter since
valid mocked interfaces are expected to start with an upper letter.
The source file is provided via --source(-file)=.... It can also be
provided by a command line argument matching the regular expression
^[a-z.].*\.go$.
The target package is provided via --target(-pkg)=.... It can also be
provided by a command line argument matching the regular expression
^[a-z_]*.
The target file is provided via --target(-file)=.... It can be also be
provided by a command line argument matching the regular expression
^[a-z.].*/mock_[^/]*\.go).