Move NIX_BIN_DIR and all logic using it to the Nix executable itself

This is because with the split packages of the Meson build, we simply
have no idea what directory the binaries will be installed in when we
build the library.

In the process of doing so, consolidate and make more sophisticated the
logic to cope with a few corner cases (e.g. `NIX_BIN_DIR` exists, but no
binaries are inside it).

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/executable-path.cc

@@ -60,7 +60,7 @@ OsString ExecutablePath::render() const
 }
 
 std::optional<fs::path>
-ExecutablePath::find(const OsString & exe, std::function<bool(const fs::path &)> isExecutable) const
+ExecutablePath::findName(const OsString & exe, std::function<bool(const fs::path &)> isExecutable) const
 {
     // "If the pathname being sought contains a <slash>, the search
     // through the path prefixes shall not be performed."
@@ -76,4 +76,20 @@ ExecutablePath::find(const OsString & exe, std::function<bool(const fs::path &)>
     return std::nullopt;
 }
 
+fs::path ExecutablePath::findPath(const fs::path & exe, std::function<bool(const fs::path &)> isExecutable) const
+{
+    // "If the pathname being sought contains a <slash>, the search
+    // through the path prefixes shall not be performed."
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+    if (exe.filename() == exe) {
+        auto resOpt = findName(exe, isExecutable);
+        if (resOpt)
+            return *resOpt;
+        else
+            throw ExecutableLookupError("Could not find executable '%s'", exe.native());
+    } else {
+        return exe;
+    }
+}
+
 } // namespace nix