Allow skipping imported resource files from export

[KoBeWi]

Fixes #67844 (it technically resolves the issue in a different way)

Imported resources go through a different code path, so _export_file() is not called for them. However they have a separate customization code, so I just allowed to use skip() when customizing to skip the resource.

The only problem is that customization has some cache, so the skip may be ignored if the file was cached and customization does not modify it. But it still at least makes it possible to skip these files, so that's good enough perhaps.

[akien-mga]

Thanks!

[allenwp]

I recently made a docs PR to describe the behaviour of _export_file and it seemed wrong to believe the design intent for this user-facing scripting API was to exclude imported resources, while including all other resources. It was a little tricky for me to wrap my head around which files are called in which EditorExportPlugin functions without some trial-and-error.

Additionally, there is a (possibly extremely minor) issue when skipping scenes in _customize_scene.

All this got me thinking about whether this PR really is the proper fix for the original issue regarding imported resources not being included in _export_file calls. Is it possible that something a little like this could be the solution for this issue instead of allowing skipping during the customize resource/scenes calls? I believe that the API would be a bit more reasonable if skipping was only performed in _export_file and all files were included in _export_file.

My branch is something I put together in 10 minutes, so I expect there are some issues with it; my intent is just to describe the sort of change I'm proposing.

[KoBeWi]

tbh this is what I wanted to do at first, I just wasn't sure how and went with a simpler solution. I tested your commit and it correctly skips files, so you can open a PR.

[allenwp]

tbh this is what I wanted to do at first, I just wasn't sure how and went with a simpler solution. I tested your commit and it correctly skips files, so you can open a PR.

Thanks! I'm working on the new PR now.