Allow users to pass arguments to `buildDotnetModule` in the form:
```nix
buildDotnetModule (finalAttrs: {
# Args
})
```
Exposing the behaviour of the underlying `mkDerivation` and allowing
packages to be defined in a recursive way that works correctly even when
the package is overridden, e.g. using `overrideAttrs`.
Added some simple test cases that piggyback on the existing
`structured-attrs` test.
This change refactors internal hooks used by buildDotnetModule to
support derivations with structured attributes. Note that this changes
variable names that the internal hooks expect.
