Perform tilde expansion when completing flake fragments

Allows completing `nix build ~/flake#<Tab>`.
We can implement expansion for `~user` later if needed.
Not using wordexp(3) since that expands way too much.

src/libutil/util.hh

@@ -68,6 +68,9 @@ Path dirOf(const PathView path);
    following the final `/' (trailing slashes are removed). */
 std::string_view baseNameOf(std::string_view path);
 
+/* Perform tilde expansion on a path. */
+std::string expandTilde(std::string_view path);
+
 /* Check whether 'path' is a descendant of 'dir'. Both paths must be
    canonicalized. */
 bool isInDir(std::string_view path, std::string_view dir);