build: repair the clean build on Windows

[compnerd]

The previous changes fixed the warnings, but resulted in the in ability
to find libxml2 on Windows. This undoes part of the previous changes,
and mostly enables building this package on Windows.

The remaining issue resolves around the link time dependency handling on
Windows.

[compnerd]

Sorry about the churn here @mattt. There is one more issue that I need to figure out how to handle, so this change on its own is insufficient still unfortunate, but does stand on its own. This repairs the build on Windows with SPM. I think that this should be fine to merge (it simply reverts part of my previous changes to Package.swift). However, I would request that you don't cut a release with this just yet. I would like to figure out the libxml2 situation before cutting a new release.

[mattt]

The previous changes fixed the warnings, but resulted in the inability to find libxml2 on Windows.

Well, that's one way to silence warnings 😆

No worries at all about the churn. I'll a cut a new release once I get your other PR merged.

I'm also looking into adding CI coverage for Windows. This is being tracked by #7.