Rather than disallowing `ext:` resolution, clear the module map after
initializing extensions so extension modules are anonymized. This
operation is explicitly called in `deno_runtime`. Re-inject `node:`
specifiers into the module map after doing this.
Fixes #17717.
Fixes #18979.
This changes the predicate for allowing `ext:` specifier resolution from
`snapshot_loaded_and_not_snapshotting` to `ext_resolution_allowed` which
is only set to true during the extension module loading phase. Module
loaders as used in core
are now declared as `ExtModuleLoader` rather than `dyn ModuleLoader`.
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
