Rename `SearchPath`

[Ericson2314]

As is typical on that platform, Windows defines a SearchPath macro to choose between SearchPathA and SearchPathW. The macro conflicts with our struct name.

Originally posted by @Ericson2314 in #8901 (comment)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-on-windows/1113/109

[roberth]

NixSearchPath, or do we have an alternate term for this concept?
Doesn't hurt to qualify it anyway. There's other search paths out there, like PATH, etc, and we interact with those too.

[Ericson2314]

LookupPath is an option, since the docs also use that.

I would like to avoid Nix since it is already in the Nix namespace. Similarly, I regret putting Nix in NixStringContext, which was a silly mistake on my part.

[roberth]

Color me α-indifferent.
I'll take what the PR brings.