Distinguish between HPATH, SPATH and plain PATH

[sternenseemann]

Description

Paths are currently just parsed into parts of strings and identifiers (in case of interpolation). Nix has three distinct types of paths that need to be treated differently (at least they are in evaluation):

• Regular paths may be subject to path interpolation. They also need may need to be resolved to absolute paths. We could be very precise and distinguish between absolute and relative paths (Upstream C++ Nix doesn't), but probably path libraries are in general well equipped to correctly to consume both relative and absolute paths and ensure that they are absolute…
• Home relative paths may be subject to path interpolation. They need to be resolved relative to the current user's home directory.
• NIX_PATH expressions (SPATH tokens) need to be resolved based on the configured nix search path (usually via the NIX_PATH environment variable). They are not subject to path interpolation.

It would be great if they were distinct on the type level. Currently, you need to parse what rnix gives you again to figure out what kind of path you are dealing with.