improve elektraPluginFactory

[markus2330]

Furthermore, I propose (maybe even the bigger change in terms of work) that we drop the name "modules". Instead, only the name "plugin" should be used for plugins, even on loader level.

Since elektraModulesLoad already returns a elektraPluginFactory that is a sensible change.

Additionally, I would also change elektraPluginFactory from

typedef Plugin * (*elektraPluginFactory) (void);

to

typedef KeySet * (*elektraPluginFactory) (void);

where the returned KeySet * is the contract of the Plugin to avoid the hacky overload of the get function.
I think that is already mentioned in another issue. This also requires changing elektraPluginExport and elektraPluginOpen, but should be doable without any other changes.

Originally posted by @kodebach in #3486 (comment)

[github-actions]

I mark this stale as it did not have any activity for one year. I'll close it in two weeks if no further activity occurs. If you want it to be alive again, ping by writing a message here or create a new issue with the remainder of this issue.
Thank you for your contributions 💖

[github-actions]

I closed this now because it has been inactive for more than one year. If I closed it by mistake, please do not hesitate to reopen it or create a new issue with the remainder of this issue.
Thank you for your contributions 💖