Improve support for subflakes

Subflakes are flakes in the same tree, accessed in flake inputs via
relative paths (e.g. `inputs.foo.url = "path:./subdir"`). Previously
these didn't work very well because they would be separately copied to
the store, which is inefficient and makes references to parent
directories tricky or impossible. Furthermore, they had their own NAR
hash in the lock file, which is superfluous since the parent is
already locked.

Now subflakes are accessed via the accessor of the calling flake. This
avoids the unnecessary copy and makes it possible for subflakes to
depend on flakes in a parent directory (so long as they're in the same
tree).

Lock file nodes for relative flake inputs now have a new `parent` field:

  {
    "locked": {
      "path": "./subdir",
      "type": "path"
    },
    "original": {
      "path": "./subdir",
      "type": "path"
    },
    "parent": [
      "foo",
      "bar"
    ]
  }

which denotes that `./subdir` is to be interpreted relative to the
directory of the `bar` input of the `foo` input of the root flake.

Extracted from the lazy-trees branch.

src/libfetchers/fetchers.cc

@@ -141,6 +141,12 @@ bool Input::isLocked() const
     return scheme && scheme->isLocked(*this);
 }
 
+std::optional<std::string> Input::isRelative() const
+{
+    assert(scheme);
+    return scheme->isRelative(*this);
+}
+
 Attrs Input::toAttrs() const
 {
     return attrs;