fix: Add a virtual destructor to DependencyProvider

[jjerphan]

When subclassing DependencyProvider, I currently get this warning:

warning: base class 'struct resolvo::DependencyProvider' has accessible non-virtual destructor [-Wnon-virtual-dtor]
   53 | struct PackageDatabase : public DependencyProvider {
      |        ^~~~~~~~~~~~~~~

I think any base class which has at least one virtual function, must have virtual destructor to respect class polymorphism.

[baszalmstra]

Can you explain why that is needed? The code that interfaces with the provider doesnt destruct it, does it?

[baszalmstra]

Nvm, did my research! 🧐 LGTM!