This has a few benefits:
* As system installed extensions become more ubiquitous, we can optionally
build against those rather than including them in the source tree,
without any source changes
* No need to hack extension headers to change the include paths to match
our specific scheme (i.e. headers are precisely those from the extension,
even if they include other extension headers)
* Consistency, lack of ambiguity, easy code sharing, blah blah, etc.
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