Revert #1135

[XAMPPRocky]

Hey, I upgraded my mdbook and my project stopped building when using mdbook-0.3.6 because of #1135. I'm fairly sure its behaviour is incorrect and should be reverted. For some context, in rust-lang teams and projects groups have a meta repository for team coordination (e.g. rust-lang/lang-team). Some of us also started trying to have this information available through mdbook. As it can be a nicer interface at times especially since it shows a complete tree of contents that you control where as GitHub does not. These repositories are entirely markdown files and doesn't really make sense to have every folder be a subfolder in the src directory.

You can see in the gh-pages branch of rust-lang/project-group-template it doesn't generate recursive copies of itself, it generates a normal mdbook project. To me this is a legitimate behaviour that should be allowed.

[ehuss]

Sorry, I'm a bit confused. Is it just not working correctly, or is it giving an error message? It might be helpful to provide a reproduction, and an explanation of what isn't behaving as expected. From the given links, I see a template of a book with 4 chapters, and I don't see anything particularly wrong with it.

[XAMPPRocky]

@ehuss Sorry, I wrote this late and forgot to include the logs. The project fails to build with Destination directory cannot be contained in source directory error. https://github.com/rust-lang/project-group-template/runs/556822171#step:3:12