I can easily get it through using shell commands which is not idempotent

You can't really talk about idempotence, when you are querying the current state of a machine.

"Idempontent" means that the task will ensure the machine is in the desired state no matter how many times you run a certain task.

When you query current state, you don't describe the desired state. No matter what you do, what method you use, the term "idempotent" is just not applicable.

Regarding your example, which does not give you results - you have repeated twice the same argument list and the task should fail (it doesn't, which looks like an Ansible quirk).

To get a list of installed packages, you should use:

- name: yum_command 
  yum:
    list=installed
  register: yum_packages

- debug:
    var: yum_packages

It saves a list of dictionaries describing each package to a variable yum_packages.

You can then use a JSON Query Filter to get a single package (tar):

- debug: var=item
  with_items: "{{yum_packages|json_query(jsonquery)}}"
  vars:
    jsonquery: "results[?name=='tar']"

to get a result like this:

"item": {
    "arch": "x86_64",
    "epoch": "2",
    "name": "tar",
    "nevra": "2:tar-1.26-31.el7.x86_64",
    "release": "31.el7",
    "repo": "installed",
    "version": "1.26",
    "yumstate": "installed"
}

Or only its version:

- debug: var=item
  with_items: "{{yum_packages|json_query(jsonquery)}}"
  vars:
    jsonquery: "results[?name=='tar'].version"

"item": "1.26"

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

Building up on @gary lopez answer to add security and performance.

First you will need to get an actual list of all packages you want to see installed on your final machine, including the default ones that come with the system. I assume that list will be in var yum_rpm

Once you have that, the next step is to get the list of currently installed packages on the machine. To create an actual list we can reuse:

  - name: Get installed packages
    yum:
      list: installed
    register: __yum_packages

  - name: Make installed packages a list of names
    set_fact:
      installed_packages: "{{ __yum_packages.results | map(attribute='name') | list }}"

From there, adding and removing is just a matter of making a difference on lists. The goal here is to avoid looping on the yum module package by package (because it is damn slow and listed as a bad practice on the module documentation page) and to make the install and remove operations in one go.

  - name: align packages on system to expected
    yum:
      name: "{{ item.packages }}"
      state: "{{ item.state }}"
    loop:
      - packages: "{{ yum_rpm | difference(installed_packages) }}"
        state: present
      - packages: "{{ installed_packages | difference(yum_rpm) }}"
        state: absent
    when: item.packages | length > 0