Issue Description:

When a package manifest includes a package with a `pkgConfig` property, it’s evidently possible for local installations to be missing the specific `.pc` files. When that happens, we show a “couldn't find pc file” warning in the package resolution log, but that’s it — we should improve this warning and provide more context (e.g., which file couldn’t we find and why were we looking for it).

Example manifest, excerpted from the attached project:

```
let package = Package(
name: "swift-nio-zlib-support",
pkgConfig: "zlib")
```

In the case of zlib, installing the pkg-config CLI tool resolved the issue.

I think we should at least have a more descriptive name instead of "pc file" and should show the path(s) we looked at and for which file.