makepkg: refactor check_sanity, give it some sanity of its own

Break apart each of the blocks into their own separate functions. And,
instead of the hand crafted eval statements, reuse the logic from
pkgbuild-introspection[0] to abstract away the complexities of parsing
bash.

This commit fixes at least 3 bugs in check_sanity:

1) The wrong variable is shown for the error which would be thrown
when, e.g.  pkgname=('foopkg' 'bar^pkg')
2) The "arch" variable is not sanity checked when the PKGBUILD has
an arch override, but only one output package.
3) https://bugs.archlinux.org/task/40361

Lastly, there's some string changes here which should help to clarify
a few errors emitted in the linting process.

[0] https://github.com/falconindy/pkgbuild-introspection

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