DUB: Sometimes generates .di file and sometimes do not.

[BoQsc]

**Conclusive observation:**
This can be resolved by informing `dub` of `extraDependencyFiles` 
to rebuild itself.
`library.di` non-existance simply does not trigger rebuild of the 
package and instead only `library.lib` is being copied from the 
`cache` folder into `build` folder. Leaving `library.di` not 
being generated.

if `cache` folder exists with a output file, it does not trigger 
build, only copies from the `\cache`'s `\build` folder. If build 
is not triggered, `.di` file is not being generated.

### Observations:
* Happens with `dub.json` as well.
* Mostly `.di` stops generating after the first output of `dub`.
* Making any text changes to content of `dub.sdl` triggers `dub` 
to rebuild package and generate`.di`

* Deleting the `cache` output file will trigger to rebuild the 
package and generate `.di`
    * 
`C:\Users\Windows10\AppData\Local\dub\cache\dheaders\~master\build`
    * After deleting `library.lib` inside latest cache folder 
`build`: The `.di` starts to generate.



### Solution:
> [**excludedSourceFiles**](https://dub.pm/package-format-sdl#build-settings)   Files that should be removed for the set of already added source files (takes precedence over "sourceFiles" and "sourcePaths") - Glob matching can be used to pattern match multiple files at once



### Working example:

**dub.sdl**

```
name "dheaders"
description "generates .di header file for a static library."
authors "public domain"
copyright "Public Domain. No rights reserved."
license "public domain"

configuration "staticLibrary" {
	dflags "-Hf=$PACKAGE_DIR/builds/library.di"
	targetName "library"
	targetType "library"
	targetPath "builds"
	sourceFiles "library.d"	
	extraDependencyFiles "$PACKAGE_DIR/builds/library.di"
}
```