[SPARK-29293][BUILD] Move scalafmt to Scala 2.12 profile; bump to 0.12

### What changes were proposed in this pull request?

Move scalafmt to Scala 2.12 profile; bump to 0.12.

### Why are the changes needed?

To facilitate a future Scala 2.13 build.

### Does this PR introduce any user-facing change?

None.

### How was this patch tested?

This isn't covered by tests, it's a convenience for contributors.

Closes apache#26655 from srowen/SPARK-29293.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

dev/scalafmt

@@ -20,4 +20,4 @@
 # by default, format only files that differ from git master
 params="${@:---diff}"
 
-./build/mvn mvn-scalafmt_2.12:format -Dscalafmt.skip=false -Dscalafmt.parameters="$params"
+./build/mvn -Pscala-2.12 mvn-scalafmt_2.12:format -Dscalafmt.skip=false -Dscalafmt.parameters="$params"

pom.xml

@@ -2843,24 +2843,6 @@
           </execution>
         </executions>
       </plugin>
-      <plugin>
-        <groupId>org.antipathy</groupId>
-        <artifactId>mvn-scalafmt_2.12</artifactId>
-        <version>0.9_1.5.1</version>
-        <configuration>
-          <parameters>${scalafmt.parameters}</parameters> <!-- (Optional) Additional command line arguments -->
-          <skip>${scalafmt.skip}</skip> <!-- (Optional) skip formatting -->
-          <configLocation>dev/.scalafmt.conf</configLocation> <!-- (Optional) config locataion -->
-        </configuration>
-        <executions>
-          <execution>
-            <phase>validate</phase>
-            <goals>
-              <goal>format</goal>
-            </goals>
-          </execution>
-        </executions>
-      </plugin>
       <!--
         Couple of dependencies are coming in bundle format (bundle is just a normal jar which
         contains OSGi metadata in the manifest). If one don't use OSGi, then a bundle will work as
@@ -3020,9 +3002,35 @@
       </properties>
     </profile>
 
-    <!-- Exists for backwards compatibility; profile doesn't do anything -->
     <profile>
       <id>scala-2.12</id>
+      <build>
+        <pluginManagement>
+          <plugins>
+            <!-- SPARK-29293 currently not able to update to 1.x for Scala 2.13 -->
+            <plugin>
+              <groupId>org.antipathy</groupId>
+              <artifactId>mvn-scalafmt_2.12</artifactId>
+              <version>0.12_1.5.1</version>
+              <configuration>
+                <parameters>${scalafmt.parameters}</parameters> <!-- (Optional) Additional command line arguments -->
+                <skip>${scalafmt.skip}</skip> <!-- (Optional) skip formatting -->
+                <skipSources>${scalafmt.skip}</skipSources>
+                <skipTestSources>${scalafmt.skip}</skipTestSources>
+                <configLocation>dev/.scalafmt.conf</configLocation> <!-- (Optional) config location -->
+              </configuration>
+              <executions>
+                <execution>
+                  <phase>validate</phase>
+                  <goals>
+                    <goal>format</goal>
+                  </goals>
+                </execution>
+              </executions>
+            </plugin>
+          </plugins>
+        </pluginManagement>
+      </build>
     </profile>
 
     <profile>