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/fetchers.cc

@@ -45,12 +45,8 @@ static void fixupInput(Input & input)
     // Check common attributes.
     input.getType();
     input.getRef();
-    if (input.getRev())
-        input.locked = true;
     input.getRevCount();
     input.getLastModified();
-    if (input.getNarHash())
-        input.locked = true;
 }
 
 Input Input::fromURL(const ParsedURL & url, bool requireTree)
@@ -140,6 +136,11 @@ bool Input::isDirect() const
     return !scheme || scheme->isDirect(*this);
 }
 
+bool Input::isLocked() const
+{
+    return scheme && scheme->isLocked(*this);
+}
+
 Attrs Input::toAttrs() const
 {
     return attrs;
@@ -222,8 +223,6 @@ std::pair<StorePath, Input> Input::fetch(ref<Store> store) const
                 input.to_string(), *prevRevCount);
     }
 
-    input.locked = true;
-
     return {std::move(storePath), input};
 }