Fix splitter on fsx files.
Fix splitter on fsx files.

The getMainScript function was only working when splitting fsproj
files.

When it wasn't given a project, there was "lastFile" to create the
entry point for.

This change uses the "entry" script as the name when there are not
any projectFiles available - assuming it must be a single FSX.

fable-compiler/Fable,ncave/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,inosik/Fable,inosik/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,ncave/Fable,fable-compiler/Fable,fable-compiler/Fable,ncave/Fable,ncave/Fable,ncave/Fable