Build: introduce javaPluginConvention extension on project

`ExtensionAware.the<T>()` introduced in gradle 4.7 made existing calls `the<JavaPluginConvention>()` invalid (on wrong receiver)

prepare/compiler-client-embeddable/build.gradle.kts

@@ -42,7 +42,7 @@ projectTest {
               ":kotlin-script-runtime:dist")
     workingDir = File(rootDir, "libraries/tools/kotlin-compiler-client-embeddable-test/src")
     doFirst {
-        systemProperty("kotlin.test.script.classpath", the<JavaPluginConvention>().sourceSets.getByName("test").output.classesDirs.joinToString(File.pathSeparator))
+        systemProperty("kotlin.test.script.classpath", javaPluginConvention().sourceSets.getByName("test").output.classesDirs.joinToString(File.pathSeparator))
         systemProperty("compilerJar", testRuntimeCompilerJar.singleFile.canonicalPath)
         systemProperty("stdlibJar", testStdlibJar.singleFile.canonicalPath)
         systemProperty("scriptRuntimeJar", testScriptRuntimeJar.singleFile.canonicalPath)

prepare/compiler/build.gradle.kts

@@ -37,7 +37,7 @@ val compilerModules: Array<String> by rootProject.extra
 compilerModules.forEach { evaluationDependsOn(it) }
 
 val compiledModulesSources = compilerModules.map {
-    project(it).the<JavaPluginConvention>().sourceSets.getByName("main").allSource
+    project(it).javaPluginConvention().sourceSets.getByName("main").allSource
 }
 
 dependencies {

prepare/compiler/build.gradle.kts.173

@@ -37,7 +37,7 @@ val compilerModules: Array<String> by rootProject.extra
 compilerModules.forEach { evaluationDependsOn(it) }
 
 val compiledModulesSources = compilerModules.map {
-    project(it).the<JavaPluginConvention>().sourceSets.getByName("main").allSource
+    project(it).javaPluginConvention().sourceSets.getByName("main").allSource
 }
 
 dependencies {

prepare/compiler/build.gradle.kts.182

@@ -37,7 +37,7 @@ val compilerModules: Array<String> by rootProject.extra
 compilerModules.forEach { evaluationDependsOn(it) }
 
 val compiledModulesSources = compilerModules.map {
-    project(it).the<JavaPluginConvention>().sourceSets.getByName("main").allSource
+    project(it).javaPluginConvention().sourceSets.getByName("main").allSource
 }
 
 dependencies {

prepare/formatter/build.gradle.kts

@@ -10,7 +10,7 @@ runtimeJar {
     dependsOn(":idea:formatter:classes")
     project(":idea:formatter").let { p ->
         p.pluginManager.withPlugin("java") {
-            from(p.the<JavaPluginConvention>().sourceSets.getByName("main").output)
+            from(p.javaPluginConvention().sourceSets.getByName("main").output)
         }
     }
     from(fileTree("$rootDir/idea/formatter")) { include("src/**") } // Eclipse formatter sources navigation depends on this

prepare/ide-lazy-resolver/build.gradle.kts

@@ -32,7 +32,7 @@ runtimeJar {
     dependsOn(":idea:ide-common:classes")
     project(":idea:ide-common").let { p ->
         p.pluginManager.withPlugin("java") {
-            from(p.the<JavaPluginConvention>().sourceSets.getByName("main").output)
+            from(p.javaPluginConvention().sourceSets.getByName("main").output)
         }
     }
     from(fileTree("$rootDir/idea/ide-common")) { include("src/**") } // Eclipse formatter sources navigation depends on this