refactor: rename InternalModuleLoader to ExtModuleLoader, use ext: scheme for snapshotted modules (#18041)

This commit renames "deno_core::InternalModuleLoader" to
"ExtModuleLoader" and changes the specifiers used by the 
modules loaded from this loader to "ext:".

"internal:" scheme was really ambiguous and it's more characters than
"ext:", which should result in slightly smaller snapshot size.

Closes https://github.com/denoland/deno/issues/18020

cli/tools/coverage/mod.rs

@@ -594,7 +594,7 @@ fn filter_coverages(
   coverages
     .into_iter()
     .filter(|e| {
-      let is_internal = e.url.starts_with("internal:")
+      let is_internal = e.url.starts_with("ext:")
         || e.url.ends_with("__anonymous__")
         || e.url.ends_with("$deno$test.js")
         || e.url.ends_with(".snap");