Use -Xallow-kotlin-package while compiling common sources

K2MetadataCompiler doesn't perform explicit check of compilation of
'kotlin' package, thus stdlib build wasn't passing '-Xallow-kotlin-package'.

However, we have special hack for compiling stdlib with contracts: if
'-Xallow-kotlin-package' is passed, then allow compiling contracts for
custom functions. We have to do it this way, because we don't want
stdlib artifacts to be marked with pre-release flag, even though
contracts for custom functions are not enabled yet
(see eae9923dbe).

Therefore, it's crucial to pass -Xallow-kotlin-package properly for that
side-effect.

libraries/stdlib/common/build.gradle

@@ -57,7 +57,8 @@ compileKotlinCommon {
     kotlinOptions {
         freeCompilerArgs = [
                 "-module-name", project.name,
-                "-Xuse-experimental=kotlin.Experimental"
+                "-Xuse-experimental=kotlin.Experimental",
+                "-Xallow-kotlin-package"
         ]
     }
 }
@@ -69,7 +70,8 @@ compileUnsignedKotlinCommon {
         freeCompilerArgs += [
                 "-Xuse-experimental=kotlin.Experimental",
                 "-Xuse-experimental=kotlin.ExperimentalUnsignedTypes",
-                "-XXLanguage:+InlineClasses"
+                "-XXLanguage:+InlineClasses",
+                "-Xallow-kotlin-package"
         ]
     }
 }