Crash when importing json using skpm/builder
Hi :)
I found a problem that crashes so I will report it just in case.
I will show details of the reproduced development environment. 

Sketch version: 47.1

↓skpm/builder version
```
  "dependencies": {
    "@skpm/builder": "^0.2.1"
  }
```

↓ tree
```
src
├── manifest.json
├── my-command.js
└── sample.json
```

↓ sample.json
```
{
  "hello": "Hello!"
}
```

↓my-command.js
```
const json = require('./sample.json');
export default function(context) {
  console.info(json.hello);
}
```

↓ crash report
```
Process:               Sketch [81009]
Path:                  /Applications/Sketch.app/Contents/MacOS/Sketch
Identifier:            com.bohemiancoding.sketch3
Responsible:           Sketch [81009]
6   com.bohemiancoding.sketch3    	0x0000000105cd1ea9 0x105990000 + 3415721
       0x105990000 -        0x105f42f87 +com.bohemiancoding.sketch3 (47.1 - 45422) <24BF32F0-4F05-3D46-816F-595EC122402F> /Applications/Sketch.app/Contents/MacOS/Sketch
       0x1060ef000 -        0x106151ff7 +net.hockeyapp.sdk.mac (3.1.0 - 28) <DFDB847E-A96E-35B9-A743-79E9AAC7BE56> /Applications/Sketch.app/Contents/Frameworks/HockeySDK.framework/Versions/A/HockeySDK
       0x1061ad000 -        0x106270fff +com.cocoascript.CocoaScript (1.0) <B3865A71-581D-3185-9077-A6164E124C13> /Applications/Sketch.app/Contents/Frameworks/CocoaScript.framework/Versions/A/CocoaScript
       0x1062cc000 -        0x1062f3ff3 +net.pol-online.GCDWebServers (3.3.4 - 3.3.4) <3E4AF7C4-1606-3908-8497-85988BBC337A> /Applications/Sketch.app/Contents/Frameworks/GCDWebServers.framework/Versions/A/GCDWebServers
       0x106317000 -        0x10634eff7 +org.sparkle-project.Sparkle (1.15.0 - 1.15.0) <09554A71-6166-318F-BE94-3218E528CD16> /Applications/Sketch.app/Contents/Frameworks/Sparkle.framework/Versions/A/Sparkle
       0x106375000 -        0x106385ff7 +co.awkward.SketchCloudKit (1.0 - 1) <52E00741-5EC1-32A3-8241-F7023FEA4E57> /Applications/Sketch.app/Contents/Frameworks/SketchCloudKit.framework/Versions/A/SketchCloudKit
```