Input: Replace 'locked' bool by isLocked() method

It's better to just check whether the input has all the attributes
needed to consider itself locked (e.g. whether a Git input has an
'rev' attribute).

Also, the 'locked' field was actually incorrect for Git inputs: it
would be set to true even for dirty worktrees. As a result, we got
away with using fetchTree() internally even though fetchTree()
requires a locked input in pure mode. In particular, this allowed
'--override-input' to work by accident.

The fix is to pass a set of "overrides" to call-flake.nix for all the
unlocked inputs (i.e. the top-level flake and any --override-inputs).

src/libfetchers/github.cc

@@ -280,6 +280,11 @@ struct GitArchiveInputScheme : InputScheme
         return {accessor, input};
     }
 
+    bool isLocked(const Input & input) const override
+    {
+        return (bool) input.getRev();
+    }
+
     std::optional<ExperimentalFeature> experimentalFeature() const override
     {
         return Xp::Flakes;