Deno emit doesn't replace file extensions

[Eyal-Shalev]

Problem

1. The files struct (Record<string,string>), returned by running Deno.emit has the (semi) correct extensions for the file names (the keys of the struct), but the code inside the emitted output uses the original .ts extension for the import statements.
   This requires special handling after emitting the output to search and replace all references to the old file extensions (as using the old ones won't work).

2. I wrote above that the keys have "semi" correct extensions, because the emit method only appends .js, .js.map & .d.ts to the keys, without considering that the "real" output file shouldn't have .ts in it's name.

Example

You can see below that the output for a.ts.js has import * as b from '/b.ts'; instead of a.ts.js has import * as b from '/b.ts.js';

build.ts

const {files} = await Deno.emit(
  "/a.ts",
  {
    sources: {
      "/b.ts": "export const foo=true",
      "/a.ts": "import * as b from '/b.ts';\nconsole.log(b)"
    }
  }
);
console.log(files)

command

deno run -q --unstable build.ts

output

{
  "file:///b.ts.js.map": '{"version":3,"file":"","sourceRoot":"","sources":["file:///b.ts"],"names":[],"mappings":"AAAA,MAAM,C...',
  "file:///a.ts.js.map": '{"version":3,"file":"","sourceRoot":"","sources":["file:///a.ts"],"names":[],"mappings":"AAAA,OAAO,K...',
  "file:///a.ts.js": "import * as b from '/b.ts';\nconsole.log(b);\n",
  "file:///b.ts.js": "export const foo = true;\n"
}

[kitsonk]

This is a duplicate of denoland/deno_emit#41 and something we need to address.