Creates "from __main__" import for __main__ program

Given `t-pyannotate.py`:
```python
from pyannotate_runtime import collect_types

collect_types.init_types_collection()
collect_types.start()

class G:
pass

def c(obj):
pass

c(G())

collect_types.stop()
collect_types.dump_stats("type_info.json")
```

Running and writing it produces the following diff, where it tries to import `G` from `__main__` then:

```diff
--- t-pyannotate.py (original)
+++ t-pyannotate.py (refactored)
@@ -1,4 +1,5 @@
from pyannotate_runtime import collect_types
+from __main__ import G

collect_types.init_types_collection()
@@ -10,6 +11,7 @@

def c(obj):
+ # type: (G) -> None
pass
```

`type_info.json`:
```json
[
{
"path": "t-pyannotate.py",
"line": 8,
"func_name": "G",
"type_comments": [
"() -> None"
],
"samples": 1
},
{
"path": "t-pyannotate.py",
"line": 12,
"func_name": "c",
"type_comments": [
"(__main__.G) -> None"
],
"samples": 1
}
]
```

pyannotate b7f96ca (current master).