seal: Change prefix and module name from "ext_" to "seal_" for contract callable functions (#6798)

* seal: Change prefix "ext_" to "seal_" for contract callable functions

The word Ext is a overloaded term in the context of substrate. It usually
is a trait which abstracts away access to external resources usually in order
to mock them away for the purpose of tests. The contract module has its own
`Ext` trait in addition the the substrate `Ext` which makes things even more
confusing.

In order to differentiate the contract callable functions more clearly from
this `Ext` concept we rename them to use the "seal_" prefix instead.

This should change no behaviour at all. This is a pure renaming commit.

* seal: Rename import module from "env" to "seal0"

* seal: Fixup integration test

* seal: Add more tests for new import module names

substrate/frame/contracts/fixtures/ok_trap_revert.wat

@@ -1,6 +1,6 @@
 (module
-	(import "env" "ext_input" (func $ext_input (param i32 i32)))
-	(import "env" "ext_return" (func $ext_return (param i32 i32 i32)))
+	(import "seal0" "seal_input" (func $seal_input (param i32 i32)))
+	(import "seal0" "seal_return" (func $seal_return (param i32 i32 i32)))
 	(import "env" "memory" (memory 1 1))
 
     (func (export "deploy")
@@ -13,7 +13,7 @@
 
     (func $ok_trap_revert
         (i32.store (i32.const 4) (i32.const 4))
-        (call $ext_input (i32.const 0) (i32.const 4))
+        (call $seal_input (i32.const 0) (i32.const 4))
         (block $IF_2
             (block $IF_1
                 (block $IF_0
@@ -26,7 +26,7 @@
                 return
             )
             ;; 1 = revert
-            (call $ext_return (i32.const 1) (i32.const 0) (i32.const 0))
+            (call $seal_return (i32.const 1) (i32.const 0) (i32.const 0))
             (unreachable)
         )
         ;; 2 = trap