Throw an error if includes or data directory is outside of Eleventy’s working directory

[kleinfreund]

This pull request is the first step for #368.

• Makes TemplateConfig throw an error on construction if either the includes or data directories point outside of Eleventy’s working directory.

• There is also some unrelated clean up in the first commit:

  The TemplateWriter class no longer creates its EleventyFiles object itself when it is missing. This was only there to ensure that the EleventyFiles object is set during testing. Setting it is now a responsibility of the tests itself.

What do you think about the error message or error handling in general?

Note: Since there might be users who rely on the current functionality, this is a breaking change.

[kleinfreund]

If this looks good to you, I can do the same for the input and output directories.

[kleinfreund]

I resolved the conflicts with the latest changes in the main branch; hence, the changes needed to be force-pushed again after rebasing.

[d2s]

Good work with the initial concept. This kind of improvements increase my trust in the future of Eleventy. 👍

[kleinfreund]

I modified the first commit in the following way:

The TemplateWriter class no longer creates its EleventyFiles object itself when it is missing. This was only there to ensure that the EleventyFiles object is set during testing. Setting it is now a responsibility of the tests itself.

I’ll update the original PR message.