rustdoc: List of issues related to Markdown input files

I didn't feel like opening 3 2 GH issues for these niche issues, so here's a single one.

• Given a Markdown input file and --markdown-css file.css, the depinfo (--emit=depinfo -Zunstable-options) doesn't list file.css
• Given a Markdown input file and --markdown-css file.css, the generated HTML document contains <link rel="stylesheet" href="file.css"> but that path won't resolve unless you set -o. / --output-dir=. since it doesn't properly account for the output directory (which is doc/ by default)
• Given a Markdown input file and --emit=html-static-files or --emit=dep-info or --emit=html-static-files,dep-info, we still emit a non-static HTML file, e.g, doc/file.html (which should only be emitted if --emit contains html-non-static-files which it implicitly does by default)

Start by reproducing the remaining rustdoc Markdown-input cases with --markdown-css file.css and the listed --emit options, using the default doc/ output directory. Trace rustdoc's depinfo, stylesheet-link, and static-file emission paths; done means file.css is tracked, the stylesheet link resolves from doc/, and non-static HTML is not emitted for static-only requests.