Introducing `isInstalled` utility

[adhamfarrag]

Describe the feature

Introducing isInstalled to check if a package is installed or not.

Additional information

• Would you be willing to help implement this feature?

[pi0]

Related to #7. There was an initial PR but i wanted to iterate overit together with local+remote checks. I think we can make this utility on top of that.

[adhamfarrag]

@pi0 What's missing from the initial PR? What should be returned to users when getPackageInfo('nypm') ?

[pi0]

(it is something i need to investigate honestly before suggesting an API so i guess have to finally pick it myself in order to iterate faster, ideas welcome in meantime)