Flakes: refetch the input when a follows disappears

When an input follows disappears, we can't just reuse the old lock
file entries since we may be missing some required ones. Refetch the
input when this happens.

Closes https://github.com/NixOS/nix/issues/5289

tests/flakes.sh

@@ -707,10 +707,10 @@ cat > $flakeFollowsA/flake.nix <<EOF
         B = {
             url = "path:./flakeB";
             inputs.foobar.follows = "D";
+            inputs.nonFlake.follows = "D";
         };
 
         D.url = "path:./flakeD";
-        foobar.url = "path:./flakeE";
     };
     outputs = { ... }: {};
 }
@@ -720,7 +720,8 @@ cat > $flakeFollowsB/flake.nix <<EOF
 {
     description = "Flake B";
     inputs = {
-        foobar.url = "path:./../flakeE";
+        foobar.url = "path:$flakeFollowsA/flakeE";
+        nonFlake.url = "path:$nonFlakeDir";
         C = {
             url = "path:./flakeC";
             inputs.foobar.follows = "foobar";
@@ -734,7 +735,7 @@ cat > $flakeFollowsC/flake.nix <<EOF
 {
     description = "Flake C";
     inputs = {
-        foobar.url = "path:./../../flakeE";
+        foobar.url = "path:$flakeFollowsA/flakeE";
     };
     outputs = { ... }: {};
 }
@@ -765,6 +766,27 @@ nix flake lock $flakeFollowsA
 [[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '["D"]' ]]
 [[ $(jq -c .nodes.C.inputs.foobar $flakeFollowsA/flake.lock) = '["B","foobar"]' ]]
 
+# Ensure removing follows from flake.nix removes them from the lockfile
+
+cat > $flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+            inputs.nonFlake.follows = "D";
+        };
+        D.url = "path:./flakeD";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+nix flake lock $flakeFollowsA
+
+[[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '"foobar"' ]]
+jq -r -c '.nodes | keys | .[]' $flakeFollowsA/flake.lock | grep "^foobar$"
+
 # Ensure a relative path is not allowed to go outside the store path
 cat > $flakeFollowsA/flake.nix <<EOF
 {