feat: add helper function: get_dependency_specs and test

[xiangce]

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Complete Description of Additions/Changes:

Add a helper function to the insights.core.dr module to get the dependency specs from a loaded component.

[xiangce]

Test Coverage:

# py.test --cov insights/core/ | egrep "dr.py|---|Cover   Missing"
Name                               Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------------
insights/core/dr.py                  483     53    204     21    87%   109->exit, 177->exit, 204, 255, 279, 285-286, 292-293, 298-301, 305, 
                                                                       340, 454-456, 460->exit, 465, 473, 483, 487->481, 523->exit, 560, 688, 701, 745, 805, 809, 816-819, 855-856, 869, 875, 893, 898-900, 915, 925, 927, 961, 969-972, 999->exit, 1034-1036, 1082-1088

This function locates in L364 ~ L424. Its coverage reaches 100%.