protogen(yaml): only read schema documents for templating data document

[wenchy]

• close #118

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.50980% with 13 lines in your changes missing coverage. Please review.

Project coverage is 67.11%. Comparing base (e116e35) to head (9a676eb).

Files	Patch %	Lines
internal/importer/yaml.go	74.50%	7 Missing and 6 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #122 +/- ## ========================================== + Coverage 66.96% 67.11% +0.14% ========================================== Files 44 44 Lines 5552 5598 +46 ========================================== + Hits 3718 3757 +39 + Misses 1355 1354 -1 - Partials 479 487 +8 ```

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