Fix checking for @_implementationOnly support (#409)
Fix checking for @_implementationOnly support (#409)

This was incorrectly passing the source file as a params file producing
this error:

```
DEBUG: .../swift_autoconfiguration.bzl:65:5: 1 <unknown>:0: error: no such file or directory: '@_implementationOnly'
<unknown>:0: error: no such file or directory: 'import'
<unknown>:0: error: no such file or directory: 'Foundation'
<unknown>:0: error: no such file or directory: 'print(Hello)'
```
bazelbuild/rules_swift,bazelbuild/rules_swift