Cannot add extensions to Gradle project with comma-separated extensions list
**Describe the bug**
In the guide: https://quarkus.io/guides/gradle-tooling#dealing-with-extensions

it looks like we can add extensions in a comma-separated list like so: 
`./gradlew addExtension --extensions="quarkus-artemis-jms,quarkus-vertx"`

However, for my Gradle project, I get this message:
```
> Task :addExtension
❌ Cannot find a dependency matching 'quarkus-artemis-jms,quarkus-vertx', maybe a typo?
```

I took those extensions from the list provided with ./gradlew listExtensions.

**Expected behavior**
Should be able to install each extension in the comma-separated list

**Actual behavior**
I get the error message above


**To Reproduce**
Steps to reproduce the behavior:
1. Create a new Gradle project using the Quarkus Maven plugin (as explained in the guide)
2. Create Gradle wrapper using `gradle wrapper` (I'm using Gradle 5.6.4)
3. Run `./gradlew addExtension --extensions="quarkus-artemis-jms,quarkus-vertx"`

Step 2 is optional, the extensions don't install whether or not I use the wrapper.

**Environment (please complete the following information):**
 - Output of `uname -a` or `ver`: 
Linux unused-10-15-17-2.yyz.redhat.com 5.0.17-300.fc30.x86_64 #1 SMP Mon May 20 15:36:26 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

 - Output of `java -version`: 
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-b04)
OpenJDK 64-Bit Server VM (build 25.212-b04, mixed mode)

 - Quarkus version or git rev: 
0.28.1
diff --git a/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusAddExtension.java b/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusAddExtension.java
index ebb5051c2cc..9639f52ccfe 100644
--- a/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusAddExtension.java
+++ b/devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusAddExtension.java
@@ -1,7 +1,9 @@
 package io.quarkus.gradle.tasks;
 
+import static java.util.Arrays.stream;
+import static java.util.stream.Collectors.toSet;
+
 import java.io.IOException;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
@@ -37,10 +39,13 @@ public List<String> getExtensionsToAdd() {
 
     @TaskAction
     public void addExtension() {
-
         setupPlatformDescriptor();
 
-        Set<String> extensionsSet = new HashSet<>(getExtensionsToAdd());
+        Set<String> extensionsSet = getExtensionsToAdd()
+                .stream()
+                .flatMap(ext -> stream(ext.split(",")))
+                .map(String::trim)
+                .collect(toSet());
         try {
             new AddExtensions(new GradleBuildFile(new FileProjectWriter(getProject().getProjectDir())))
                     .addExtensions(extensionsSet);
['devtools/gradle/src/main/java/io/quarkus/gradle/tasks/QuarkusAddExtension.java']