unable to install fslint

"The following packages have unmet dependencies: youtube-downloader"

It seems that the dependency problem was there already before the attempt to install fslint.

For diagnostic purposes please provide the output that you receive for the commands:

uname -a
lsb_release -crid
sudo dpkg --audit
apt-cache policy fslint youtube-downloader qml-module-qtquick-controls libqt5quickwidgets5 qt5-image-formats-plugins

command : uname -a
output:-
Linux shaifali-hp-notebook 4.15.0-88-generic #88~16.04.1-Ubuntu SMP Wed Feb 12 04:19:15 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

command :lsb_release -crid
output :
Distributor ID: Ubuntu
Description: Ubuntu 16.04.3 LTS
Release: 16.04
Codename: xenial

command : sudo dpkg --audit
output:
[sudo] password for ankit:
The following packages have been unpacked but not yet configured.
They must be configured using dpkg --configure or the configure
menu option in dselect for them to work:
 youtube-downloader YouTube Downloader

command : apt-cache policy fslint youtube-downloader qml-module-qtquick-controls libqt5quickwidgets5 qt5-image-formats-plugins
output :
fslint:
  Installed: (none)
  Candidate: 2.44-2
  Version table:
     2.44-2 500
        500 http://in.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        500 http://in.archive.ubuntu.com/ubuntu xenial/universe i386 Packages
youtube-downloader:
  Installed: 3.9.9.59
  Candidate: 3.9.9.59
  Version table:
 *** 3.9.9.59 100
        100 /var/lib/dpkg/status
qml-module-qtquick-controls:
  Installed: (none)
  Candidate: 5.5.1-1ubuntu1
  Version table:
     5.5.1-1ubuntu1 500
        500 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
libqt5quickwidgets5:
  Installed: (none)
  Candidate: 5.5.1-2ubuntu6
  Version table:
     5.5.1-2ubuntu6 500
        500 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
qt5-image-formats-plugins:
  Installed: (none)
  Candidate: 5.5.1-2build1
  Version table:
     5.5.1-2build1 500
        500 http://in.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages

Support for Ubuntu 16.04 as desktop operating system has ended almost a year ago (it is in ESM status which is a paid support extension for servers). You should upgrade to a supported release in due course.

For the current problem: What output do you receive for the command
sudo apt install youtube-downloader qml-module-qtquick-controls libqt5quickwidgets5 qt5-image-formats-plugins

command : sudo apt install youtube-downloader qml-module-qtquick-controls libqt5quickwidgets5 qt5-image-formats-plugins
output :
[sudo] password for ankit:
Reading package lists... Done
Building dependency tree
Reading state information... Done
youtube-downloader is already the newest version (3.9.9.59).
The following packages were automatically installed and are no longer required:
  libuv1 linux-headers-4.13.0-39 linux-headers-4.13.0-39-generic linux-image-4.13.0-39-generic linux-image-extra-4.13.0-39-generic
  linux-signed-image-4.13.0-39-generic
Use 'sudo apt autoremove' to remove them.
The following NEW packages will be installed:
  libqt5quickwidgets5 qml-module-qtquick-controls qt5-image-formats-plugins
0 upgraded, 3 newly installed, 0 to remove and 701 not upgraded.
1 not fully installed or removed.
Need to get 737 kB of archives.
After this operation, 3,321 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 qml-module-qtquick-controls amd64 5.5.1-1ubuntu1 [643 kB]
Get:2 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 libqt5quickwidgets5 amd64 5.5.1-2ubuntu6 [30.4 kB]
Get:3 http://in.archive.ubuntu.com/ubuntu xenial/universe amd64 qt5-image-formats-plugins amd64 5.5.1-2build1 [64.0 kB]
Fetched 737 kB in 1s (423 kB/s)
Selecting previously unselected package qml-module-qtquick-controls:amd64.
(Reading database ... 309339 files and directories currently installed.)
Preparing to unpack .../qml-module-qtquick-controls_5.5.1-1ubuntu1_amd64.deb ...
Unpacking qml-module-qtquick-controls:amd64 (5.5.1-1ubuntu1) ...
Selecting previously unselected package libqt5quickwidgets5:amd64.
Preparing to unpack .../libqt5quickwidgets5_5.5.1-2ubuntu6_amd64.deb ...
Unpacking libqt5quickwidgets5:amd64 (5.5.1-2ubuntu6) ...
Selecting previously unselected package qt5-image-formats-plugins:amd64.
Preparing to unpack .../qt5-image-formats-plugins_5.5.1-2build1_amd64.deb ...
Unpacking qt5-image-formats-plugins:amd64 (5.5.1-2build1) ...
Processing triggers for libc-bin (2.23-0ubuntu10) ...
Setting up qml-module-qtquick-controls:amd64 (5.5.1-1ubuntu1) ...
Setting up libqt5quickwidgets5:amd64 (5.5.1-2ubuntu6) ...
Setting up qt5-image-formats-plugins:amd64 (5.5.1-2build1) ...
Setting up youtube-downloader (3.9.9.59) ...
Processing triggers for libc-bin (2.23-0ubuntu10) ...

Thanks Manfred Hampl, that solved my question.