Description:
When a package only supports one arch but has nominally arch-specific sources (e.g.: when we are just repackaging an arch-dependent binary from upstream, but upstream only provides binary for one of the arches supported by Arch Linux), it makes sense to specify the sources as "source_$arch" (e.g. "source_x86" instead of "source" (this usage is consistent with the "Tip" at https://wiki.archlinux.org/index.php/PKGBUILD#source). But namcap specifically throws a warning in that case; I consider the warning invalid.


Additional info:
* namcap 3.2.7


Steps to reproduce:
Create the following PKGBUILD

# Maintainer: me.
pkgname='foobar'
pkgdesc='hello'
url='https://not.an.url'
license=('MIT')
pkgver=0
pkgrel=0
arch=('x86_64')
source_x86_64=('foobar.baz')

build() {
true
}
package() {
true
}

and pass it to namcap. It prints:
PKGBUILD (foobar) W: Non standard variable 'source_x86_64' doesn't start with an underscore


The relevant line is at https://git.archlinux.org/namcap.git/tree/Namcap/rules/extravars.py#n35, where arch-specific variables are considered only if `len(pkginfo["arch"]) >= 2`. Deleting that clause should be sufficient; happy to provide a patch if that helps.