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/src/benchmarking.rs

@@ -130,9 +130,9 @@ benchmarks! {
 
 	// Instantiate uses a dummy contract constructor to measure the overhead of the instantiate.
 	// The size of the data has no influence on the costs of this extrinsic as long as the contract
-	// won't call `ext_input` in its constructor to copy the data to contract memory.
+	// won't call `seal_input` in its constructor to copy the data to contract memory.
 	// The dummy contract used here does not do this. The costs for the data copy is billed as
-	// part of `ext_input`.
+	// part of `seal_input`.
 	instantiate {
 		let data = vec![0u8; 128];
 		let endowment = Config::<T>::subsistence_threshold_uncached();