tools: Add dependecy download helpers

This patch adds 2 helpers to download dependencies for compiler and
stub generator during their execution.

They take list of external dependencies as a parameter and directory
where they must be located. Helpers check if these dependencies exist
in the given directory and download not existing ones.

buildSrc/src/main/groovy/org/jetbrains/kotlin/KonanTest.groovy

@@ -12,7 +12,7 @@ abstract class KonanTest extends JavaExec {
     def backendNative = project.project(":backend.native")
     def runtimeProject = project.project(":runtime")
     def dist = project.rootProject.file("dist")
-    def dependencies = project.findProject(":dependencies").file("all")
+    def dependenciesDir = project.findProject(":dependencies").file("all")
     def runtimeBc = new File("${dist.canonicalPath}/lib/runtime.bc").absolutePath
     def launcherBc = new File("${dist.canonicalPath}/lib/launcher.bc").absolutePath
     def startKtBc = new File("${dist.canonicalPath}/lib/start.kt.bc").absolutePath
@@ -59,7 +59,7 @@ abstract class KonanTest extends JavaExec {
     void setJvmArgs(Iterable<?> arguments) {
         super.setJvmArgs(arguments +
                 "-Dkonan.home=${dist.canonicalPath}" +
-                "-Dkonan.dependencies=${dependencies.canonicalPat}",
+                "-Dkonan.dependencies=${dependenciesDir.canonicalPath}" +
                 "-Djava.library.path=${dist.canonicalPath}/konan/nativelib")
     }