Resolves transformerPath before attempting to read contents. (#268)

Summary:
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**Summary**
Configuration paths are now supposed to be relative to allow them to be shared
on different paths/machines. This change makes sure that the new relative
transformerPath is require.resolved before being passed to fs.readFileSync.
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**Test plan**
I edited node_modules directly in metro-sample-app to ensure that the fix got
me passed the unable to resolve error.
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Pull Request resolved: #268

Differential Revision: D10062783

Pulled By: rafeca

fbshipit-source-id: 3f4e97adf1153987c609d97e951cf3d16ea18fda

packages/metro/src/DeltaBundler/Transformer/getTransformCacheKey.js

@@ -47,7 +47,7 @@ function getTransformCacheKey(opts: {|
     VERSION,
     opts.cacheVersion,
     path.relative(path.join(__dirname, '../../../..'), opts.projectRoot),
-    getKeyFromFiles([transformerPath]),
+    getKeyFromFiles([require.resolve(transformerPath)]),
     transformerKey,
   ];