[Assign plugin] Use FqName to match annotations in FirAssignAnnotationMatchingService

All other compiler plugins accept "pkg1.pkg2.Class1" classes notation
and correctly match against it

But Assignment plugin used `ClassId` on such qualified names and got
`ClassId("/pkg1.pkg2.Class1")` instead of `ClassId("pkg1/pkg2/Class1")`,
and that lead to unexpected problems

This commit fixes that by using `FqName` instead of `ClassId`

^KT-57406 Fixed

plugins/assign-plugin/testData/codegen/qualifiedAnnotation.kt

@@ -0,0 +1,22 @@
+// FILE: annotation.kt
+package qualified
+
+annotation class ValueContainer
+
+// FILE: main.kt
+import qualified.ValueContainer
+
+@ValueContainer
+class StringProperty(var v: String) {
+    fun assign(v: String) {
+        this.v = v
+    }
+}
+
+val property = StringProperty("Initial")
+
+fun box(): String {
+    property = "OK"
+
+    return property.v
+}