Found a behaviour mismatch: when an array is provided in exports field,
enhanced-resolve will try to read the file and try the next one if it
fails. But in the spec, the path is not read, it's only tested against
"invalid package target".
```
For each item targetValue in target, do
1. Let resolved be the result of PACKAGE_TARGET_RESOLVE( packageURL, targetValue, patternMatch, isImports, conditions), continuing the loop on any Invalid Package Target error.
2. If resolved is undefined, continue the loop.
3. Return resolved.
```
I tested against node.js and verifies node.js conforms to the spec.
