I don't think the yum module would help in this case. It currently has 3 states: absent, present, and latest. Since it sounds like you don't want to actually install or remove the package (at least at this point) then you would need to do this in two manual steps. The first task would check to see if the package exists, then the second task would invoke a command based on the output of the first command.

If you use "rpm -q" to check if a package exists then the output would look like this for a package that exists:

# rpm -q httpd
httpd-2.2.15-15.el6.centos.1.x86_64

and like this if the package doesn't exist:

# rpm -q httpdfoo
package httpdfoo is not installed

So your ansible tasks would look something like this:

- name: Check if foo.rpm is installed
  command: rpm -q foo.rpm
  register: rpm_check

- name: Execute script if foo.rpm is not installed
  command: somescript
  when: rpm_check.stdout.find('is not installed') != -1

The rpm command will also exit with a 0 if the package exists, or a 1 if the package isn't found, so another possibility is to use:

when: rpm_check.rc == 1

I just want to update this old discussion to point out that there is now a package module that makes this more straightforward

- name: get the rpm or apt package facts
  package_facts:
    manager: "auto"

- name: show apache2 version
  debug: var=ansible_facts.packages.apache2[0].version

You don't need to check if the package is installed or not before doing anything, this is what the state: present is for:

present and installed will simply ensure that a desired package is installed.

^{Source: https://docs.ansible.com/ansible/latest/collections/ansible/builtin/yum_module.html#parameter-state}

So your playbook should be as simple as

- name: Check Package installation
  hosts: "{{ exec_server }}"
  become: yes
  gather_facts: no
  
  tasks:
    - name: install the latest version of not installed Packages
      yum:
        name: "{{ pkgs }}"
        state: present 

Now if you want to know what is incorrect in your construct, it is the if:

{% if pkgs is undefined in ansible_facts.packages %}-{{ pkgs }}{% endif %}

You cannot run two test on the same variable in the same instruction (without an or or and, that is).
While here you are trying to run an in and is undefined tests at the same time.

What you could do, if you want to keep in that route, which is sub-optimal, as stated above, is to use the difference filter:

- name: install the latest version of not installed Packages
  yum:
    name: "{{ pkgs | difference(ansible_facts.packages) }}"
    state: latest