[K/N] Refactor :kotlin-native:dependencies

Move K/N dependencies handling logic into gradle plugins:

`NativeDependenciesDownloader` is instantiated in
`:kotlin-native:dependencies` and provides a single configuration that
contains all directories with the dependencies. The configuration has
a variant for each target.

`NativeDependencies` is instantiated by projects that require native
dependencies (i.e. previously used to depend on
`:kotlin-native:dependencies:update`). This plugin creates a configuration
that by default depends on `:kotlin-native:dependencies`. Its extension
provides `llvmDependency`, `libffiDependency`, `hostPlatformDependency`,
`targetDependency()` to depend on the parts needed for the project and
provides accessors `llvmPath`, `libffiPath`, `hostPlatform` that are
safe to use during configuration time, but which do not automatically
resolve those dependencies.

`llvmDir`, `*LibffiDir` properties on `:kotlin-native` are removed
completely, as their use is replaced by `NativeDependencies`. As a
consequence, `evaluationDependsOn(":kotlin-native:dependencies")` is
also gone.

All direct dependencies on `:kotlin-native:dependencies:update` are
replaced with dependencies on specific parts via `NativeDependencies`

kotlin-native/Interop/StubGenerator/build.gradle.kts

@@ -21,6 +21,7 @@ buildscript {
 plugins {
     kotlin("jvm")
     application
+    id("native-dependencies")
 }
 
 application {
@@ -59,11 +60,11 @@ tasks {
                 project(":kotlin-native:Interop:Runtime")
         )
         dependsOn(projectsWithNativeLibs.map { "${it.path}:nativelibs" })
+        dependsOn(nativeDependencies.llvmDependency)
         systemProperty("java.library.path", projectsWithNativeLibs.joinToString(File.pathSeparator) {
             File(it.buildDir, "nativelibs").absolutePath
         })
-        val llvmDir = project.findProperty("llvmDir")
-        val libclangPath = "$llvmDir/" + if (org.jetbrains.kotlin.konan.target.HostManager.hostIsMingw) {
+        val libclangPath = "${nativeDependencies.llvmPath}/" + if (org.jetbrains.kotlin.konan.target.HostManager.hostIsMingw) {
             "bin/libclang.dll"
         } else {
             "lib/${System.mapLibraryName("clang")}"