Add support to read classpath from the CLASSPATH environment variable (#256)

* Support to use CLASSPATH environment variable

* Add tests

Author: andirady

pom.xml

@@ -160,6 +160,10 @@
                         --add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED
                         --add-opens jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
                     </argLine>
+                    <environmentVariables>
+                        <!-- used by InferConfigTest.java -->
+                        <CLASSPATH>src/test/examples/home-dir/.m2/repository/com/external/external-library/1.1/external-library-1.1.jar</CLASSPATH>
+                    </environmentVariables>
                 </configuration>
             </plugin>
             <!-- Copy dependencies to dist -->

src/main/java/org/javacs/InferConfig.java

@@ -10,6 +10,7 @@
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.*;
+import java.util.stream.*;
 import java.util.logging.Logger;
 import java.util.regex.Pattern;
 import java.util.stream.Stream;
@@ -78,6 +79,14 @@ Set<Path> classPath() {
             return bazelClasspath(bazelWorkspaceRoot);
         }
 
+        var classpathEnvVar = System.getenv("CLASSPATH");
+        LOG.fine(() -> "CLASSPATH=" + classpathEnvVar);
+        if (classpathEnvVar != null) {
+            var paths = Stream.of(classpathEnvVar.split(System.getProperty("path.separator"))).map(Path::of).collect(Collectors.toSet());
+            LOG.fine(() -> "Paths from CLASSPATH = " + paths);
+            return paths;
+        }
+
         return Collections.emptySet();
     }
 

src/test/java/org/javacs/InferConfigTest.java

@@ -16,6 +16,7 @@ public class InferConfigTest {
     private InferConfig both = new InferConfig(workspaceRoot, externalDependencies, mavenHome, gradleHome);
     private InferConfig gradle = new InferConfig(workspaceRoot, externalDependencies, Paths.get("nowhere"), gradleHome);
     private InferConfig thisProject = new InferConfig(Paths.get("."), Set.of());
+    private InferConfig simpleProject = new InferConfig(Paths.get("src/test/examples/simple-project"), Set.of());
 
     @Test
     public void mavenClassPath() {
@@ -35,6 +36,18 @@ public void gradleClasspath() {
         // v1.1 should be ignored
     }
 
+    @Test
+    public void envVarClasspath() {
+        // CLASSPATH is set in pom.xml
+        assertThat(
+                simpleProject.classPath(),
+                contains(mavenHome.resolve("repository/com/external/external-library/1.1/external-library-1.1.jar")));
+        assertThat(
+                both.classPath(),
+                not(contains(mavenHome.resolve("repository/com/external/external-library/1.1/external-library-1.1.jar"))));
+    }
+
+
     @Test
     public void mavenDocPath() {
         assertThat(