Set up a layered build for CI similar to what EW does

scripts/fetchdep.sh

@@ -0,0 +1,73 @@
+#!/usr/bin/env bash
+
+# Clones the branch of a dependency's repository that has the same name as
+# the pull request branch under test, so a change spanning both repositories
+# can be tested before either side is merged. Inspired by scripts/fetchdep.sh 
+# element-web.
+#
+# Usage: scripts/fetchdep.sh <org> <repo> [ref]
+#
+# The clone lands in ./<repo>. When the pull request comes from a fork and
+# PR_HEAD_REPO_OWNER was set, the fork's own copy of the repository is tried
+# before <org>'s. Exits 0 when a branch was cloned and 1 when there is none,
+# so the caller can keep the locked version in that case. Without a ref,
+# nothing is closed outside a pull request.
+
+set -eu
+
+org=$1
+repo=$2
+ref=${3:-}
+
+rm -rf "$repo"
+
+# If a ref was passed, try to clone it or die.
+if [ -n "$ref" ]; then
+    echo "fetchdep.sh: Using $org/$repo at $ref"
+    git clone --depth 1 "https://github.com/$org/$repo.git" "$repo"
+    git -C "$repo" fetch --depth 1 origin "$ref"
+    git -C "$repo" -c advice.detachedHead=false checkout FETCH_HEAD
+    git -C "$repo" log -1
+    exit 0
+fi
+
+# Otherwise ensure that we're on a pull request.
+head_ref=${GITHUB_HEAD_REF:-}
+if [ -z "$head_ref" ]; then
+    echo "fetchdep.sh: Not a pull request, nothing to match"
+    exit 1
+fi
+
+# Bots name their branches the same way in every repository, so a match on
+# one of theirs would be a coincidence rather than a paired change.
+case "$head_ref" in
+    renovate/* | dependabot/*)
+        echo "fetchdep.sh: Not matching bot branch $head_ref"
+        exit 1
+        ;;
+esac
+
+# Clones a branch if it exists and ends the script on success.
+clone() {
+    local try_org=$1
+    local branch=$2
+    echo "fetchdep.sh: Trying to use $try_org/$repo#$branch"
+    # Disable auth prompts: https://serverfault.com/a/665959
+    if GIT_TERMINAL_PROMPT=0 git clone "https://github.com/$try_org/$repo.git" "$repo" --branch "$branch" --depth 1; then
+        git -C "$repo" log -1
+        # Surface the match on the workflow run summary.
+        echo "::notice title=Matched $repo branch::Using $try_org/$repo#$branch at $(git -C "$repo" rev-parse HEAD)"
+        exit 0
+    fi
+}
+
+# Try cloning from the fork, then from the org.
+head_owner=${PR_HEAD_REPO_OWNER:-}
+if [ -n "$head_owner" ] && [ "$head_owner" != "${GITHUB_REPOSITORY_OWNER:-}" ]; then
+    clone "$head_owner" "$head_ref"
+fi
+clone "$org" "$head_ref"
+
+# Nothing worked. Die.
+echo "fetchdep.sh: No branch named $head_ref in $org/$repo"
+exit 1