build: silence an annoying warning on Windows

[compnerd]

This alteration to Package.swift mirrors the changes that were done in
the SwiftDocOrg/Markup package. It removes the pkg-config, brew
providers to silence some warnings when building on Windows. These
tools are normally not distributed on Windows, and require that the user
manually pass along the graphviz installation path.

This is preparatory work, there is further fixes required to enable the
use of this package on Windows.

[compnerd]

@mattt this is similar to the changes in SwiftDocOrg/Markup to silence the warning due to the use of pkg-config and brew. There is a more complicated issue which arises from the graphviz libraries that I have yet to figure out how to resolve properly. However, this does make it easier to make progress.

[mattt]

@compnerd Thanks for this PR. I apologize for inadvertently making swift-doc so difficult to deploy by incorporating so many C library dependencies. I really appreciate your taking the time to make this work, and want nothing more than for us to get this working on Windows.