dependencies in requirements.txt not present in top level

[AyanSinhaMahapatra]

See the test here for which top-level dependencies doesn't contain the requirements that are present in this package_data dependencies

This was probably introduced here

[AyanSinhaMahapatra]

See discussion here

[swastkk]

After reviewing the discussion on aboutcode-org/scancode.io#516 (comment),
what i get is
we have the class PipRequirementsFileHandler for handling the requirments file with the PKG-INFO handler, but the difficulty we are facing is getting the results from the both simulatanously when the package contains the both and list them in the dependencies section.

also we need to have a check for the duplicate dependencies coming from the PKG-INFO and RequirementHandler and report a single unique one in the package_data dependencies.