Including SQL files in extension scripts

In writing extension update scripts, I find it to be really difficult to
grok diffs for example in changed view or function definitions when a new
extension script has to include the whole definition in a new file. I want
to rather use separate files for these objects, then use something like
psql's \i to include the definitions so that version control (and
management of these files) becomes easier, but this does not appear to be
supported. Does anyone have any recommendations here?

Currently, I often simply opt to create a script that builds out the
extension SQL files from separated sql files so that I can more easily
maintain them. But I was hoping there is a better way. Any ideas would be
much appreciated.

Thanks,
Jeremy