Fix isUpToDate for sub-packages

`packageInfoFile` of sub-packages are not used(it contains just `Path.init`), but `BuildGenerator.isUpToDate` checks it.
I changed `BuildGenerator.isUpToDate` to look at basePackage's one.

This PR fixes problem like this:
```
...
File '.' modified, need rebuild.
Building gtk-d:gtkd ~Gtk2 configuration "library", build type debug.
...
```
For sub-packages, everytime this message was printed and made unnecessary builds.
It occured because `Path.init` means '.'