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.

backend.native/tests/build.gradle

@@ -25,8 +25,8 @@ dependencies {
     }
 }
 
-def testOutputRoot = rootProject.file("test.output").absolutePath
-def externalTestsDir = project.file("external")
+ext.testOutputRoot = rootProject.file("test.output").absolutePath
+ext.externalTestsDir = project.file("external")
 externalTestsDir.mkdirs()
 
 allprojects {