[ffigen] Fix leaker health check

[liamappelbe]

A lot of internal classes are being leaked to the public API, causing spurious breaking change reports:

  "breakingChanges": {
    "label": "BREAKING CHANGES",
    "children": [
      {
        "label": "Class ImportedType",
        "children": [
          {
            "label": "Constructor ImportedType",
            "children": [
              {
                "changeDescription": "Kind of parameter \"defaultValue\" changed. not named -> named",
                "changeCode": "CE07",
                "isBreaking": true,
                "type": "major"
              }
            ]
          }
        ]
      }
    ]
  },

[dcharkes]

A lot of internal classes are being leaked to the public API

Shouldn't we fix that instead? Is it not fixable? Why not? How do we have to change the code to do so?

(Edit: We should probably have the FFIgen major version bump on (a) breaking changes to FFIgen output, (b) breaking changes to the yaml config, and (c) breaking changes to the public Dart API.)

[liamappelbe]

Shouldn't we fix that instead? Is it not fixable? Why not? How do we have to change the code to do so?

You mean instead of #1858? I will fix it (that's what this bug is tracking), but I wanted to undo the unnecessary version bump first. I haven't investigated the leaks yet, but there's a lot of them and I don't know how much of a refactor it will be. I'll fix it in the next milestone.