feat: only error on unaccessible unexported fields, funcs or methods

[jmattheis]

With this feature, goverter doesn't error when using unexported funcs, methods or map unexported struct fields when they are accessible inside the output:package.

See #104

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (57ea90b) 97.84% compared to head (2f6c4a9) 97.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #107 +/- ## ========================================== + Coverage 97.84% 97.98% +0.13% ========================================== Files 39 40 +1 Lines 1766 1786 +20 ========================================== + Hits 1728 1750 +22 + Misses 26 24 -2 Partials 12 12 ```

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