Pass --external-sources to shellcheck

[rasky]

Many shell scripts use "source" to include other scripts. This option
tells shellcheck to parse those external scripts, so that the remainder
of the script can be properly linted.

[kaste]

Can't find when they added this flag nor any further descriptions 🙄 Any downsides to just add unconditional for everyone? Otherwise makes sense and lgtm.

[braver]

It's a flag to enable this behavior, so to me that says it's not supposed to be on all the time. I don't feel like having to figure out if there are downsides, or having to take it out again at a later stage because it causes problems for someone. If we bake it in like this, you can't take it out. There is also no need to bake it in: if you want this behavior you can configure args for the linter. So I don't really see a good reason to merge this PR. If I'm wrong, please let me know!

[stdedos]

Can't find when they added this flag nor any further descriptions 🙄 Any downsides to just add unconditional for everyone? Otherwise makes sense and lgtm.

See koalaman/shellcheck#902.
Easier and more admin-friendly, as always, is the .shellcheckrc file (but it does not work for a long time, koalaman/shellcheck#1811)

[kaste]

Since for example ale and other IDEs set this as explained in koalaman/shellcheck#902 we can do this too. Though instead of hard-coding it into cmd, doing it via defaults users have the chance to opt-out. For example

defaults = {
	...
	"--external-sources": True,
}

is probably the better patch.