Now that patchqueue was removed, we can consider supporting the parallel
read with our local extensions.
Since we only have basic extensions, not storing any data on the build
environment, the change mainly consists of specifying the extensions as
parallelizable.
The only specific case is the html domain, since the base html domain
requires the support of the method merge_domaindata in parallel read
mode. Since we do not need to share anything between the envs for this
extension, we can simply ignore the method.
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