Overloading `build_style` for meta-packages has always felt very hacky,
and it prevents having the main package be a meta-package (with non-meta
subpackages) and using a `build_style` at the same time (example of
trying to hack around that: 62c0c08e18).
It also can create confusion about how subpackages work if you can
define a `build_style` there (but only `meta`).
This makes using `build_style=meta` an error at the package and subpackage
level, and replaces it with a new variable (`metapackage=yes`).
A pkglint for non-empty meta-packages is also added.
classabbyamp
479bef3b2f