Clarify symbols visibility

[martinbonnin]

Right now:

• tasks are generated as internal
• workers are generated as private
• entrypoints are generated as public

This is happening regardless of whether the @GTask functions are public or not and regardless of whether classloader isolation is enabled or not.

Questions:

• Should we allow generating tasks as "public" so that they could be used by consumers? I tend to consider the tasks implementation detail but maybe that could be useful?
• Should the entrypoints always be internal? They are only called from reflection so doesn't matter much I think?
• How does that all work with Java visibility? Can we prevent Java/Groovy callers to access the internals?
• Should everything just be private abstract?