Imports block detection needs to be smarter

The current imports block detection need to be improved, for example, this is a typical and common import block in applications that supports several versions of some library or different versions of python even:

``` python
try:
import ujson as json
except ImportError:
import json

from collections import defaultdict
import sys

def main():
print(os.uname())
a = defaultdict(list)
```

This is translated to

``` python
import json
import sys
from collections import defaultdict

import ujsonas json

try:
import ujson as json
except ImportError:
import json

from collections import defaultdict
import sys

def main():
print(os.uname())
a = defaultdict(list)
```

That is a complete disaster, in the other hand, the scope is fine:

```
print(scope.find_unresolved_and_unreferenced_symbols())
(set(['os.uname']), set(['sys', 'json', 'main']))
```

The importer has no clue about what is really imported in the file as its not able to know where the imports block really ends.