Reapply "libfetchers: make attribute / URL query handling consistent"

The original attempt at this introduced a regression; this commit
reverts the revert and fixes the regression.

This reverts commit 3e151d4d77.

Fix to the regression:

flakeref: fix handling of `?dir=` param for flakes in subdirs

As reported in #419[1], accessing a flake in a subdir of a Git
repository fails with the previous commit[2] applied with the error

    error: unsupported Git input attribute 'dir'

The problem is that the `dir`-param is inserted into the parsed URL if a
flake is fetched from the subdir of a Git repository. However, for the
fetching part this isn't even needed. The fix is to just pass `subdir`
as second argument to `FlakeRef` (which needs a `basedir` that can be
empty) and leave the parsedURL as-is.

Added a regression test to make sure we don't run into this again.

[1] https://git.lix.systems/lix-project/lix/issues/419
[2] e22172aaf6b6a366cecd3c025590e68fa2b91bcc,
    originally 3e151d4d77

Change-Id: I2c72d5a32e406a7ca308e271730bd0af01c5d18b

src/libexpr/flake/flakeref.cc

@@ -169,14 +169,13 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
                         if (subdir != "") {
                             if (parsedURL.query.count("dir"))
                                 throw Error("flake URL '%s' has an inconsistent 'dir' parameter", url);
-                            parsedURL.query.insert_or_assign("dir", subdir);
                         }
 
                         if (pathExists(flakeRoot + "/.git/shallow"))
                             parsedURL.query.insert_or_assign("shallow", "1");
 
                         return std::make_pair(
-                            FlakeRef(Input::fromURL(parsedURL, isFlake), getOr(parsedURL.query, "dir", "")),
+                            FlakeRef(Input::fromURL(parsedURL, isFlake), subdir),
                             fragment);
                     }
 
@@ -204,7 +203,13 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
         std::string fragment;
         std::swap(fragment, parsedURL.fragment);
 
-        auto input = Input::fromURL(parsedURL, isFlake);
+        // This has a special meaning for flakes and must not be passed to libfetchers.
+        // Of course this means that libfetchers cannot have fetchers
+        // expecting an argument `dir` 🫠
+        ParsedURL urlForFetchers(parsedURL);
+        urlForFetchers.query.erase("dir");
+
+        auto input = Input::fromURL(urlForFetchers, isFlake);
         input.parent = baseDir;
 
         return std::make_pair(