Use stable identifiers for generated CSS class names
Currently, there is typically a lot of work to do after upgrading between
compound-web releases, in updating test snapshots. This is because CSS classes
are named based on a hash of the class module's *contents*.
Currently, classes have names like `._text-content_zx76t_44`, where `text-button`
is the original css name, `zx76t` is a hash of the css module file, and `44` is
the line number within the file.
We switch to names like `_cpd-src-components-Alert-_text-content`, where
`src-components-Alert-` is the path to the CSS module (with `/` replaced by
`-`).
