Dub will now automatically exclude `mainSourceFile` from other configurations

By default, Dub uses all files it can find under its `sourcePaths`.
However, a common pattern when dealing with multiple targets is to use
configurations to represent said targets. In the case those targets are executables,
users would be forced to add main files from other configurations to the
`excludedSourceFiles` list, or store the main in a different directory outside of
the sourcePaths.

To simplify this workflow, Dub will now exclude files listed in mainSourceFile
for other configuration. In case this is not desirable, the files need to be manually
added to the `sourceFiles` list.