apt build-dep fails, unable to locate source package, despite deb-src lines present in the /etc/apt/sources.list.d folder

I ran sudo apt-get build-dep emacs24 and it returned the error:

Reading package lists... Done
E: Unable to find a source package for emacs24

Now I know the usual advice in this situation, that is, uncomment deb-src lines in the .list files in your /etc/apt/sources.list.d folder (per https://unix.stackexchange.com/questions/158395/apt-get-build-dep-is-unable-to-find-a-source-package). I've tried this though! Every single file in my /etc/apt/sources.list.d folder has any deb-src lines in it uncommented. I even ran sudo apt-get update after uncommenting these lines! So please help me. Here are the contents of my /etc/apt/sources.list.d folder, if you need the contents of any of these files, just say and I'll be happy to provide them.

$ ls /etc/apt/sources.list.d
alexlarsson-ubuntu-flatpak-xenial.list
alexlarsson-ubuntu-flatpak-xenial.list.save
cpick-ubuntu-hub-xenial.list
cpick-ubuntu-hub-xenial.list.save
damien-moore-ubuntu-codeblocks-stable-xenial.list
damien-moore-ubuntu-codeblocks-stable-xenial.list.save
danielrichter2007-ubuntu-grub-customizer-xenial.list
danielrichter2007-ubuntu-grub-customizer-xenial.list.save
docker.list
docker.list.save
dolik-rce-ubuntu-upp-xenial.list
dolik-rce-ubuntu-upp-xenial.list.save
dotnetdev.list
dotnetdev.list.save
gnome3-team-ubuntu-gnome3-staging-xenial.list
gnome3-team-ubuntu-gnome3-staging-xenial.list.save
gnome3-team-ubuntu-gnome3-xenial.list
gnome3-team-ubuntu-gnome3-xenial.list.save
google-chrome.list
google-chrome.list.save
google-earth.list
google-earth.list.save
icons.list
icons.list.save
jonathonf-ubuntu-golang-xenial.list
jonathonf-ubuntu-golang-xenial.list.save
kubuntu-ppa-ubuntu-backports-xenial.list
kubuntu-ppa-ubuntu-backports-xenial.list.save
mmk2410-ubuntu-eclipse-ide-java-xenial.list
mmk2410-ubuntu-eclipse-ide-java-xenial.list.save
mono-jenkins.list
mono-jenkins.list.save
mono-xamarin.list
mono-xamarin.list.save
nodesource.list
nodesource.list.save
noobslab-ubuntu-macbuntu-xenial.list
noobslab-ubuntu-macbuntu-xenial.list.save
notepadqq-team-ubuntu-notepadqq-xenial.list
notepadqq-team-ubuntu-notepadqq-xenial.list.save
runescape.list
runescape.list.save
saiarcot895-ubuntu-myppa-xenial.list
saiarcot895-ubuntu-myppa-xenial.list.save
spotify.list
spotify.list.save
trinity.list
trinity.list.save
ubuntu-elisp-ubuntu-ppa-xenial.list
ubuntu-elisp-ubuntu-ppa-xenial.list.save
unity8-desktop-session-team-ubuntu-unity8-preview-lxc-xenial.list
wine-ubuntu-wine-builds-xenial.list
wine-ubuntu-wine-builds-xenial.list.save

If relevant, and you can't tell by the previous bits of information I provided, I am operating on Ubuntu 16.04.1.

You need to uncomment the deb-src lines in the main sources file /etc/apt/sources.list, not the extra files in /etc/apt/sources.list.d

For example, change:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted
#deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted

to

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted
deb-src http://gb.archive.ubuntu.com/ubuntu/ xenial main restricted

Then update the local database:

sudo apt update

and proceed :)

software-properties-gtk option "Source code"

From the GUI, you can run:

software-properties-gtk

then under the "Ubuntu Software" tab click "Source code":

This has added some deb-src lines under /etc/apt/sources.list, and now I can do sudo apt-get build-dep <package>.

Tested on Ubuntu 16.04 to 18.04.

CLI method

sudo sed -Ei 's/^# deb-src /deb-src /' /etc/apt/sources.list
sudo apt-get update

Tested on the Ubuntu 18.04 Docker image. After this change, I can successful do for example:

sudo apt-get build-dep hello

and before the change it failed with the error:

E: You must put some 'source' URIs in your sources.list