Add checksum check before installing a package

[rhaidiz]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?
  Add check of the package checksum when installing a package.

• What is the current behavior?
  No check on the checksum is performed.

• What is the new behavior?
  The checksum is checked before installing a new package.

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See how to contribute

[rhaidiz]

Note: since this PR adds an integrity verification when installing a package, and since the checksum and size of the core.zip file in test_index.json in the master repository is wrong, some integration tests fail.

[rsora]

@rhaidiz could you please rebase this PR on master to get the latest changes?

[rhaidiz]

Sure, PR rebased.