[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/build.gradle

@@ -79,10 +79,6 @@ ext {
 }
 
 allprojects {
-    if (path != ":kotlin-native:dependencies") {
-        evaluationDependsOn(":kotlin-native:dependencies")
-    }
-
     repositories {
         mavenCentral()
         maven {
@@ -249,7 +245,6 @@ tasks.register("distCompiler", Copy) {
     if (!UtilsKt.isDefaultNativeHome(project)) {
         enabled = false
     } else {
-        dependsOn ":kotlin-native:dependencies:update"
         dependsOn ':kotlin-native:shadowJar'
         dependsOn ":kotlin-native-compiler-embeddable:kotlin-native-compiler-embeddable"
     }