FIR: split transformer into three (imports / types / descriptors)

compiler/testData/fir/resolve/multifile/simpleAliasedImport.1.kt

@@ -0,0 +1,3 @@
+package b
+
+abstract class MyClass

compiler/testData/fir/resolve/multifile/simpleAliasedImport.kt

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+package a
+import b.MyClass as HisClass
+
+class YourClass : HisClass

compiler/testData/fir/resolve/multifile/simpleAliasedImport.txt

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+FILE: simpleAliasedImport.kt
+    unknown final class YourClass() : R/b.MyClass/ {
+    }