use-flow-type marks a reference as used with the wrong reference when names are the same

**Code**

```
import {
contextType,
} from 'someModule';

export function getLabelSets(
contextType: $Keys,
) { ... }
```

**Versions**
```
"babel-eslint": "^8.2.3",
"eslint": "^4.19.1",
"eslint-plugin-flowtype": "^2.48.0",
```

**Expected**

No errors.

**Actual**

```
error 'contextType' is defined but never used no-unused-vars
```