Type shadowing error

The following mechanism used to compile and work before but does not compile with v0.9.1:

```
module Shadow where

import Prim as P

type Number = P.Number
z :: Number
z = 0.0
```

Error:

```
Error found:
in module Shadow
at ..../Shadow.purs line 6, column 1 - line 6, column 12

  Conflicting definitions are in scope for type Number from the following modules:

    Prim
    Shadow
```
