chore: compat checks fail if a release has happened (#19017)

If a PR is branched off of a version that introduces a new
package, and lives longer than the release, the "does package
exist?" check is wrong, and fails the PR build.

This is usually fixed by another "Update from master", but that's
an annoying step that sometimes takes a while.

- Let's say the branch was forked at 1.142.0
- The branch adds a new package
- That package is released as its initial version as 1.143.0
- When the PR next builds (still at 1.142.0), it will do the following
  checks:
  - Does the package exist at all? (Answer: yes)
  - If so, try to install it at 1.142.0, because that's what we're
    diffing against (This explodes, because the package does not exist
    at that version).

Instead of doing a versionless "does the package exist at all" check,
always check the existence of the actual version we'll be diffing
against.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

scripts/check-api-compatibility.sh

@@ -14,7 +14,7 @@ package_name() {
 # Doesn't use 'npm view' as that is slow. Direct curl'ing npmjs is better
 package_exists_on_npm() {
     pkg=$1
-    ver=$2 # optional
+    ver=${2:-} # optional
     curl -I 2>/dev/null https://registry.npmjs.org/$pkg/$ver | head -n 1 | grep 200 >/dev/null
 }
 
@@ -32,10 +32,13 @@ jsii_package_dirs=$(list_jsii_packages)
 
 #----------------------------------------------------------------------
 
-# Input a directory, output the directory IF it exists on NPM
+# dirs_for_existing_pkgs DIRECTORY VERSION
+#
+# Input a directory and a version, output the directory IF it exists on NPM at that version
 dirs_for_existing_pkgs() {
     local dir="$1"
-    if package_exists_on_npm $(package_name $dir); then
+    local ver="$2"
+    if package_exists_on_npm $(package_name $dir) $ver; then
         echo "$dir"
         echo -n "." >&2
     else
@@ -49,30 +52,24 @@ export -f dirs_for_existing_pkgs
 
 if ! ${SKIP_DOWNLOAD:-false}; then
     echo "Filtering on existing packages on NPM..." >&2
-    # In parallel
-    existing_pkg_dirs=$(echo "$jsii_package_dirs" | xargs -n1 -P4 -I {} bash -c 'dirs_for_existing_pkgs "$@"' _ {})
-    existing_names=$(echo "$existing_pkg_dirs" | xargs -n1 -P4 -I {} bash -c 'package_name "$@"' _ {})
-    echo " Done." >&2
 
     echo "Determining baseline version..." >&2
     version=$(node -p 'require("./scripts/resolve-version.js").version')
     echo "  Current version is $version." >&2
+    echo "Using version '$version' as the baseline..."
 
-    if ! package_exists_on_npm aws-cdk $version; then
-        major_version=$(echo "$version" | sed -e 's/\..*//g')
-        echo "  Version $version does not exist in npm. Falling back to package major version ${major_version}" >&2
-        existing_names=$(echo "$existing_names" | sed -e "s/$/@$major_version/")
-    else
-        echo "Using version '$version' as the baseline..."
-        existing_names=$(echo "$existing_names" | sed -e "s/$/@$version/")
-    fi
+    # Filter packages by existing at the target version
+    existing_pkg_dirs=$(echo "$jsii_package_dirs" | xargs -n1 -P4 -I {} bash -c 'dirs_for_existing_pkgs "$@" "'$version'"' _ {})
+    existing_names=$(echo "$existing_pkg_dirs" | xargs -n1 -P4 -I {} bash -c 'package_name "$@"' _ {})
+    install_versions=$(for name in $existing_names; do echo "${name}@${version}"; done)
+    echo " Done." >&2
 
     rm -rf $tmpdir
     mkdir -p $tmpdir
 
     echo "Installing from NPM..." >&2
     # use npm7 to automatically install peer dependencies
-    (cd $tmpdir && npx npm@^7.0.0 install --prefix $tmpdir $existing_names)
+    (cd $tmpdir && npx npm@^7.0.0 install --prefix $tmpdir $install_versions)
 fi
 
 #----------------------------------------------------------------------