dirhtml-builder uses html-builder canonical link

[casperdcl]

Describe the bug

context jupyter-book build --builder dirhtml

expectation

<link rel="canonical" href=".../chapter/" />

bug But instead this happens:

<link rel="canonical" href=".../chapter.html" />

problem This is a problem e.g. for people using utterances comments, vis. https://github.com/utterance/utterances/issues/474

Reproduce the bug

1. jupyter-book create foo
2. Edit foo/_config.yml to add html: baseurl: http://example.com
3. jupyter-book build --builder dirhtml foo
4. grep canonical foo/_build/dirhtml/markdown/index.html

<link rel="canonical" href="http://example.com/markdown.html" />

(instead of <link rel="canonical" href="http://example.com/markdown/" />)

List your environment

$ jupyter-book --version
Jupyter Book      : 0.15.1
External ToC      : 0.3.1
MyST-Parser       : 0.18.1
MyST-NB           : 0.17.2
Sphinx Book Theme : 1.0.1
Jupyter-Cache     : 0.6.1
NbClient          : 0.7.4

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