chore: re-enable using current pkg versions for api compat checks (aw…

…s#8427)

This was originally added in commit
aws@3dd21b9.

However, the script fails during a bump build when the package version
in lerna.json is ahead of the latest published in NPM. This was worked
around by turning this feature off -
aws@09a1f33.

Re-enable this feature and handle version in lerna.json may be ahead of
NPM.

### Testing

Manually tested three cases -
* When version in `lerna.json` is <= package published in NPM
* When version in `lerna.json` is > package published in NPM
* When `DOWNLOAD_LATEST` is set to `true`.

----

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

build.sh

@@ -60,6 +60,6 @@ echo "==========================================================================
 echo "building..."
 time lerna run $bail --stream $runtarget || fail
 
-DOWNLOAD_LATEST=true /bin/bash scripts/check-api-compatibility.sh
+/bin/bash scripts/check-api-compatibility.sh
 
 touch $BUILD_INDICATOR

scripts/check-api-compatibility.sh

@@ -13,7 +13,9 @@ package_name() {
 #
 # Doesn't use 'npm view' as that is slow. Direct curl'ing npmjs is better
 package_exists_on_npm() {
-    curl -I 2>/dev/null https://registry.npmjs.org/$1 | head -n 1 | grep 200 >/dev/null
+    pkg=$1
+    ver=$2 # optional
+    curl -I 2>/dev/null https://registry.npmjs.org/$pkg/$ver | head -n 1 | grep 200 >/dev/null
 }
 
 
@@ -53,10 +55,13 @@ if ! ${SKIP_DOWNLOAD:-false}; then
     existing_names=$(echo "$jsii_package_dirs" | xargs -n1 -P4 -I {} bash -c 'dirs_to_existing_names "$@"' _ {})
     echo " Done." >&2
 
-    if ! ${DOWNLOAD_LATEST:-false}; then
-        current_version=$(node -p 'require("./lerna.json").version')
+    current_version=$(node -p 'require("./lerna.json").version')
+    echo "Current version in lerna.json is $current_version"
+    if ! ${DOWNLOAD_LATEST:-false} && package_exists_on_npm aws-cdk $current_version; then
         echo "Using package version ${current_version} as baseline"
         existing_names=$(echo "$existing_names" | sed -e "s/$/@$current_version/")
+    else
+        echo "However, using the latest version from NPM as the baseline"
     fi
 
     rm -rf $tmpdir