Clarify some return values around paths and files of `zipFunction`

[danez]

Comes from #1098

While working on the above PR, there was some confusion about naming and/or the content of certain properties that are returned by zipFunction. In detail what is inputs, srcFiles and includedFiles? The included nodejs bundlers also slightly differ in what they include in the srcFiles property.

Once we get back to this it might make sense to ...:

• Get a clear understanding what is returned by each of these properties.
• What the value of these properties is actually used for (maybe we can merge some?)
• Maybe the content is fine, but we need better names?

includedFiles was added in the PR above to enable watching files that are manually added to bundles via the config included_files.