Revise ParseFile() so that it returns "the" module if there is only module in the file
Cleanup
Merge pull request #1031 from noakesmichael/parseFile

ParseFile: Extend the cases for which a ModuleSymbol* is returned

Chapel source files have 3 important forms

1) A single top level statement that is a Module Declaration

2) Multiple top level statements that are all Module Declarations

3) All other cases e.g. a file with a variety of top level declarations and statements.

ParseFile is declared to return a ModuleSymbol* but only did so in the final case.  This "feels" wrong
and there was code elsewhere that compensated for this by other means.  Fixed the first case to also
return the ModuleSymbol*.

In a future change is might be attractive to update the API to simply return a vector of ModuleSymbols.

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/llinux64
Tested: paratest --verify
Review: none