Add step in ghc-lib guide for getting submodules to work (#11351)

changelog_begin
changelog_end

ghc-lib/new-working-on-ghc-lib.md

@@ -22,13 +22,22 @@ git remote add da-fork git@github.com:digital-asset/ghc.git
 git fetch da-fork
 ```
 
-3. Checkout the version of interest and update the submodules:
+3. Set git URL rewrite rules
+
+The `ghc` repo uses relative paths for its submodules, so things will break when using the `da-fork` remote. To avoid this, we tell git to use the address of the real submodule repos instead of the non-existent github repos it would get by following the relative paths:
+
+```
+git config --global url."ssh://git@gitlab.haskell.org/ghc/arcanist-external-json-linter.git".insteadOf git@github.com:digital-asset/arcanist-external-json-linter.git
+git config --global url."ssh://git@gitlab.haskell.org/ghc/packages/".insteadOf git@github.com:digital-asset/packages/
+```
+
+4. Checkout the version of interest and update the submodules:
 ```
 git checkout da-master-8.8.1
 git submodule update --init --recursive
 ```
 
-4. Made initial build (takes about 15 mins)
+5. Made initial build (takes about 15 mins)
 ```
 hadrian/build.stack.sh --configure --flavour=quickest -j
 ```