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/libexpr/flake/flake.hh

@@ -103,6 +103,13 @@ struct LockedFlake
     Flake flake;
     LockFile lockFile;
 
+    /**
+     * Store paths of nodes that have been fetched in
+     * lockFlake(); in particular, the root node and the overriden
+     * inputs.
+     */
+    std::map<ref<Node>, StorePath> nodePaths;
+
     Fingerprint getFingerprint() const;
 };