feat: use assignability for mapping method type compatibility validation

[jmattheis]

See #110

[codecov[bot]]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (4625b8d) 97.81% compared to head (004a569) 97.18%.

Files	Patch %	Lines
pkgload/pkgload.go	92.04%	5 Missing and 2 partials :warning:
config/config.go	57.14%	2 Missing and 1 partial :warning:
config/converter.go	85.71%	2 Missing and 1 partial :warning:
pkgload/parse.go	92.85%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #112 +/- ## ========================================== - Coverage 97.81% 97.18% -0.64% ========================================== Files 40 41 +1 Lines 2155 2168 +13 ========================================== - Hits 2108 2107 -1 - Misses 35 45 +10 - Partials 12 16 +4 ```

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