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
This commit changes "include_js_files!" macro from "deno_core"
in a way that "dir" option doesn't cause specifiers to be rewritten
to include it.
Example:
```
include_js_files! {
dir "js",
"hello.js",
}
```
The above definition required embedders to use:
`import ... from "internal:<ext_name>/js/hello.js"`.
But with this change, the "js" directory in which the files are stored
is an implementation detail, which for embedders results in:
`import ... from "internal:<ext_name>/hello.js"`.
The directory the files are stored in, is an implementation detail and
in some cases might result in a significant size difference for the
snapshot. As an example, in "deno_node" extension, we store the
source code in "polyfills" directory; which resulted in each specifier
to look like "internal:deno_node/polyfills/<module_name>", but with
this change it's "internal:deno_node/<module_name>".
Given that "deno_node" has over 100 files, many of them having
several import specifiers to the same extension, this change removes
10 characters from each import specifier.
