cli: continue converting Path to std::filesystem::path in CLI commands

[amaanq]

Motivation

This follows the previous commit by converting more string-based Path fields and locals to std::filesystem::path across the CLI layer: MixOutLinkBase::outLink, CmdAddToStore::path, CmdBundle::outLink, CmdDevelop redirects and shell, nix-build, nix-channel, nix-env, nix-instantiate, nix-store, and upgrade-nix. Also defers the std::filesystem::absolute(script.parent_path()) call in nix-build into the branch that actually needs it, since std::filesystem::absolute("") throws where the old absPath("") silently returned cwd.

Also, change runProgram and friends to use OsString for argument lists, since those will be native strings. This allows us to avoid (potentially lossy) std::filesystem::path (wide chars on windows) -> std::string (narrow) -> OsString (i.e. std::wstring on Windows, wide again) round trips. For CLIs with no path arguments, there is a toOsStrings function for convenience, however.

Context

• This builds on top of libutil: return std::filesystem::path from XDG directory helpers #15311

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[Ericson2314]

I added another commit before the main one

Replace renderUrlPathEnsureLegal with urlPathToPath

We make urlPathToPath support relative paths for of Flakes.

The mercurial fetching got a bit larger of a rework with this, by taking advantage of std::variant.

This cleaned things up a bit before the big std::filesystem::path and runProgram args push, so that itself didn't have such a big Mercurial fetching diff.