Asset subbundle builds

[guybedford]

This adds support for building emitted assets, by initiating a sub-build using the same configuration as the main build, while deduping emitted assets against the single shared asset list.

Depends on vercel/webpack-asset-relocator-loader#100 which is yet to be released, but references that from the GitHub master dependencies reference for now.

Two test cases are included one with a require and one with the typescript piscina example.

If asset builds are not wanted, a noAssetBuilds option and a --no-asset-builds flag are also provided.

[styfle]

@guybedford Looks like the tests are failing

  ● should generate correct output for bundle-subasset

    ENOENT: no such file or directory, open '/home/runner/work/ncc/ncc/test/unit/bundle-subasset/output-coverage.js'

      10 |     const testDir = `${__dirname}/unit/${unitTest}`;
      11 |     const expected = fs
    > 12 |       .readFileSync(`${testDir}/output${coverage ? '-coverage' : ''}.js`)
         |        ^
      13 |       .toString()
      14 |       .trim()
      15 |       // Windows support

      at Object.readFileSync (test/index.test.js:12:8)

  ● should generate correct output for bundle-subasset2

    ENOENT: no such file or directory, open '/home/runner/work/ncc/ncc/test/unit/bundle-subasset2/output-coverage.js'

      10 |     const testDir = `${__dirname}/unit/${unitTest}`;
      11 |     const expected = fs
    > 12 |       .readFileSync(`${testDir}/output${coverage ? '-coverage' : ''}.js`)
         |        ^
      13 |       .toString()
      14 |       .trim()
      15 |       // Windows support

[guybedford]

I've added the coverage unit test, so this should be passing now.