preGenerateCommands should reparse dub.json/dub.sdl, if needed

[WebFreak001]

When running preGenerateCommands, dub will already re-evaluate all the globs and variables from the dub recipe, however it will never actually re-parse the dub recipe. I think if the recipe has changed (modification time), we should also re-parse the recipe, so that more advanced use-cases that edit the recipe are possible.

Note that dub right now only considers itself, not dependencies or dependents when running preGenerateCommands, which is a limitation that keeps the implementation sane though.

[Geod24]

so that more advanced use-cases that edit the recipe are possible.

I don't see self-modifying code as an advanced use case, it's an error prone hack and I really don't think we should do that.