How to handle "default index" files?

[chris-dura]

A lot of static site generators allow for configuring a "default index" file, like README.md or index.md. So, markdown authors can use [link](../path/to/default/) instead of [link](../path/to/default/README.md).

I'm trying to test that the README.md file exists (and not just the default/ directory), so I tried to use replacementPatterns to assume that a path ending in a trailing slash / means it should check path/to/default/README.md.

replacementPatterns: [
  {
    // Replace URLs that start with `/` to point to `projectBaseUrl`
    pattern: '^/',
    replacement: '{{BASEURL}}/' // confusing name, but this is actually replaced by `projectBaseUrl` -_-
  },
  {
    // Replace URLs that end with `/` with `/README.md`
    // The problem with this is that it will make `https://example.com/` fail
    pattern: '/$',
    replacement: `/README.md`
  },
],

However, with the above config, it would cause legitimate external URLs like https://example.com/ to report as "dead" because it will add a README.md at the end.

- [link](../path/to/default) <!-- alive: ../path/to/default -->
- [link](../path/to/default/) <!-- alive: ../path/to/default/README.md -->
- 
- [link](http://example.com) <!-- alive -->
- [link](http://example.com/) <!-- dead: http://example.com/README.md -->

<!-- alive: ../path/to/dir/without/default (I would actually prefer this to be "dead", because the default README.md is missing) -->
- [link](../path/to/dir/with/no/default)

- [link](../path/to/dir/with/no/default/) <!-- dead: ../path/to/dir/without/default/README.md -->
- [link](../path/to/dir/with/no/default/README.md) <!-- dead: ../path/to/dir/without/default/README.md -->

Any ideas of how I can at least configure /some/path/to/default/ and https://example.com/ differently?