Build: introduce javaPluginConvention extension on project

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

libraries/kotlinx-metadata/jvm/build.gradle.kts

@@ -49,7 +49,7 @@ val shadowJar = task<ShadowJar>("shadowJar") {
     callGroovy("manifestAttributes", manifest, project)
     manifest.attributes["Implementation-Version"] = version
 
-    from(the<JavaPluginConvention>().sourceSets["main"].output)
+    from(javaPluginConvention().sourceSets["main"].output)
     exclude("**/*.proto")
     configurations = listOf(shadows)
 

libraries/reflect/build.gradle.kts

@@ -131,11 +131,11 @@ val reflectShadowJar by task<ShadowJar> {
     version = null
     callGroovy("manifestAttributes", manifest, project, "Main", true)
 
-    from(the<JavaPluginConvention>().sourceSets.getByName("main").output)
-    from(project(":core:descriptors.jvm").the<JavaPluginConvention>().sourceSets.getByName("main").resources) {
+    from(javaPluginConvention().sourceSets.getByName("main").output)
+    from(project(":core:descriptors.jvm").javaPluginConvention().sourceSets.getByName("main").resources) {
         include("META-INF/services/**")
     }
-    from(project(":core:deserialization").the<JavaPluginConvention>().sourceSets.getByName("main").resources) {
+    from(project(":core:deserialization").javaPluginConvention().sourceSets.getByName("main").resources) {
         include("META-INF/services/**")
     }
 

libraries/tools/kotlinp/build.gradle.kts

@@ -32,7 +32,7 @@ val shadowJar by task<ShadowJar> {
     classifier = "shadow"
     version = null
     configurations = listOf(shadows)
-    from(the<JavaPluginConvention>().sourceSets.getByName("main").output)
+    from(javaPluginConvention().sourceSets.getByName("main").output)
     manifest {
         attributes["Main-Class"] = "org.jetbrains.kotlin.kotlinp.Main"
     }