refactor: move minimizerReferences from files to buildFiles

- Depends on: #404

Current Nextclade versions reject array values in the `files` section of `pathogen.json` with `invalid type: sequence, expected a string`. Datasets declaring `minimizerReferences` as an array inside `files` fail to load.

Move `minimizerReferences` from `.files` to a new top-level `.buildFiles` key:

```json
{
  "files": {
    "reference": "reference.fasta",
    "genomeAnnotation": "genome_annotation.gff3",
    "changelog": "CHANGELOG.md"
  },
  "buildFiles": {
    "minimizerReferences": [
      "minimizer_refs/additional_refs_B.fasta",
      "minimizer_refs/additional_refs_B1a.fasta"
    ]
  }
}
```

Unknown top-level keys are silently ignored by Nextclade, so the dataset remains loadable by both old and new versions. The companion script change reading from `buildFiles` is in #404 (9fc7f3e, 9fc7f3e).

### Work items

- [x] Move `minimizerReferences` from `files` to `buildFiles` in `pathogen.json`

Author: ivan-aksamentov

data/enpen/enterovirus/cva16/pathogen.json

@@ -8,7 +8,9 @@
     "treeJson": "tree.json",
     "examples": "sequences.fasta",
     "readme": "README.md",
-    "changelog": "CHANGELOG.md",
+    "changelog": "CHANGELOG.md"
+  },
+  "buildFiles": {
     "minimizerReferences": [
       "minimizer_refs/additional_refs_B.fasta",
       "minimizer_refs/additional_refs_B1a.fasta",