[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/libclangext/build.gradle.kts

@@ -29,14 +29,14 @@ native {
     val obj = if (isWindows) "obj" else "o"
     val cxxflags = mutableListOf("--std=c++17", "-g",
                           "-Isrc/main/include",
-                          "-I${project.findProperty("llvmDir")}/include",
+                          "-I$llvmDir/include",
                           "-DLLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING=1")
     if (libclangextEnabled) {
         cxxflags += "-DLIBCLANGEXT_ENABLE=1"
     }
     suffixes {
         (".cpp" to ".$obj") {
-            tool(*platformManager.hostPlatform.clangForJni.clangCXX("").toTypedArray())
+            tool(*hostPlatform.clangForJni.clangCXX("").toTypedArray())
             flags(*cxxflags.toTypedArray(), "-c", "-o", ruleOut(), ruleInFirst())
         }
     }
@@ -47,7 +47,7 @@ native {
     }
     val objSet = sourceSets["main"]!!.transform(".cpp" to ".$obj")
     target(lib("clangext"), objSet) {
-        tool(*platformManager.hostPlatform.clangForJni.llvmAr("").toTypedArray())
+        tool(*hostPlatform.clangForJni.llvmAr("").toTypedArray())
         flags("-qcv", ruleOut(), *ruleInAll())
     }
 }