Relax index.Rmd constraint while still trying to guess output formats

[cderv]

This should close #1349

This PR tries to achieve two things:

• Supports index.Rmd and index.rmd as main file expected in a bookdown project
• Still render a book even if these file are not provided (basically render_book("my_file.Rmd") where my_file.Rmd plays the role of index file but with another name.

I am opening this PR as a draft to get some first feedback @yihui. I also need to check that report in community (https://community.rstudio.com/t/render-book-error-stop-if-not-exists-and-some-files-were-not-found/139927/19) is taken into account.