Specify Error Prone JVM args with .mvn/jvm.config (#2339)

* Specify Error Prone JVM args with `.mvn/jvm.config`

This allows avoiding fork mode for compilation, which:
- might slightly reduce compilation time
- guarantees that all compiler diagnostics are properly reported

* Combine enforcer executions

.mvn/jvm.config

@@ -0,0 +1,10 @@
+--add-exports jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED
+--add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED
+--add-opens jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED
+--add-opens jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED

pom.xml

@@ -104,14 +104,19 @@
  <artifactId>maven-enforcer-plugin</artifactId>
  <version>3.2.1</version>
  <executions>
- <!-- Enforce that correct JDK version is used to avoid cryptic build errors -->
  <execution>
- <id>enforce-jdk-version</id>
+ <id>enforce-versions</id>
  <goals>
  <goal>enforce</goal>
  </goals>
  <configuration>
  <rules>
+ <requireMavenVersion>
+ <!-- Usage of `.mvn/jvm.config` for Error Prone requires at least Maven 3.3.1 -->
+ <version>[3.3.1,)</version>
+ </requireMavenVersion>
+
+ <!-- Enforce that correct JDK version is used to avoid cryptic build errors -->
  <requireJavaVersion>
  <!-- Other plugins of this build require at least JDK 11 -->
  <version>[11,)</version>
@@ -133,21 +138,10 @@
  <showWarnings>true</showWarnings>
  <showDeprecation>true</showDeprecation>
  <failOnWarning>true</failOnWarning>
- <fork>true</fork>
  <compilerArgs>
  <!-- Args related to Error Prone, see: https://errorprone.info/docs/installation#maven -->
  <arg>-XDcompilePolicy=simple</arg>
  <arg>-Xplugin:ErrorProne -XepExcludedPaths:.*/generated-test-sources/protobuf/.*</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED</arg>
- <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED</arg>
- <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED</arg>
- <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED</arg>
  <!-- Enable all warnings, except for ones which cause issues when building with newer JDKs, see also
  https://docs.oracle.com/en/java/javase/11/tools/javac.html -->
  <compilerArg>-Xlint:all,-options</compilerArg>