Implement FlatDir repository for JPF core dependencies

- Add flatDir repository pointing to ../jpf-core/build
- Replace fileTree dependency with explicit JPF JAR dependencies
- Use 'implementation name: jpf-classes' and 'implementation name: jpf'
- Remove local lib/ directory approach in favor of flatDir
- Update Java compatibility to VERSION_11

This approach automatically uses the latest JARs from jpf-core
and follows Gradle best practices for local dependencies.

Author: cm45t3r

build.gradle

@@ -6,20 +6,25 @@ group = 'gov.nasa.jpf'
 version = '1.2.0'
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_1_8
-    targetCompatibility = JavaVersion.VERSION_1_8
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
 }
 
 repositories {
     mavenCentral()
+    flatDir {
+        dirs '../jpf-core/build'
+    }
 }
 
 dependencies {
-    implementation fileTree(dir: 'lib', include: '*.jar')
-    testImplementation 'junit:junit:4.13.2'
+    // JPF Core dependencies (local JARs)
+    implementation name: 'jpf-classes'
+    implementation name: 'jpf'
 
-    // CLI parsing
+    // External dependencies
     implementation 'commons-cli:commons-cli:1.5.0'
+    testImplementation 'junit:junit:4.13.2'
 }
 
 sourceSets {