Internal links to README.md are broken

[lily-commure]

README.md gets translated to index.html, but internal links (except for those in SUMMARY.md) don't get translated. So they point to a non-existent README.html path. mdBook should either generate both index.html and README.html or translate internal links so that they point to index.html instead.

[WofWca]

Ok, here's an example:
If you want to add a link to some page, you can type [Some page](some_dir/some_page.md), but if you try to link README this way, the link will be broken.

[nihaals]

Repro steps

docker run -it --rm rust:slim bash
# cargo install mdbook
# mkdir /app
# cd /app
# mdbook init
...
# printf "[Test](./test/README.md)\n[Test 2](./test.md)" > src/SUMMARY.md
# printf "[Test](./test/README.md)" > src/test.md
# mkdir src/test
# printf "# Test" > src/test/README.md
# mdbook build
# cat book/test.html | grep README.html
<p><a href="./test/README.html">Test</a></p>
# ls book/test
index.html

[YushiOMOTE]

duplicated issue? #1268

[ehuss]

Yea, both issues are essentially the same. I'll keep this one, but there is some more context and discussion in #1268 that is worth reading.