Enhance detection of conflicting destination URLs

[ashmaroli]

• This is a 🙋 feature or enhancement.
• I've added tests.

Summary

Currently on master, the build process detects conflicting URLs. But it is limited to just pages and posts.
The proposed change extends the detection to all documents and static files.
Also, improves the readability of the warning message:

   URL Conflict: The destination "jekyll/tmp/jekyll/_site/puppies/2009/03/27/rover.html" is the same for the following:
                  - jekyll/tmp/jekyll/_posts/2009-03-27-rover.markdown
                  - jekyll/tmp/jekyll/_puppies/rover.md

[ashmaroli]

@jekyllbot: merge +minor

[DirtyF]

@ashmaroli I now get a warning, that does not make sense on my personal site:

          Conflict: The following destination is shared by multiple files.
                    The written file may end up with unexpected contents.
                    /Users/frank/code/frank.taillandier.me/_site/_redirects
                     - /Users/frank/code/frank.taillandier.me/_redirects
                     - /Users/frank/code/frank.taillandier.me/_redirects

[ashmaroli]

@DirtyF Looks like this change exposed a bug present elsewhere.
Could you open a new issue ticket for this?

Preliminary investigation shows that the file ./_redirects is initialized twice. (i.e. 2 instances of Jekyll::StaticFile for the same file).
Need to investigate if a plugin is initializing the double or Jekyll core itself.