Skip module import if cursor info is missing module info

[alvarhansen]

I believe setting nextIsModuleImport = false in current code is accidental bug, as it makes no sense to set it false if it already is false. It makes more sense if it is in inner if statements else block.

At the moment it will lead to crash if you import module that SourceKit is unable to find info for.

With following code:

import UnknownModule
func foo(error: Swift.Error) {}

The execution goes like this:

1. import token is found and nextIsModuleImport is set to true
2. identifier token is found but if SourceKit returns empty dictionary the loop just continues
3. Swift token in foo(error: Swift.Error) is visited and if it has key.modulename source.lang.swift.ref.module it is falsely considered as an import statement.
4. Its name is added to unusedImports
5. contentsNSString.range(of: "import \(module)\n") returns NSRange with location NSNotFound
   ...
6. func correct(file: File, compilerArguments: [String]) -> [Correction] will throw NSException when attempting to replace this range.

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[jpsim]

Makes sense. Thanks!