Discrepancies in find_unit

[Shikhar1998]

Consider the following examples:

>>> from sympy.physics import units as u
>>> u.find_unit('charge')
    ['C', 'coulomb', 'coulombs', 'planck_charge'] # correct
>>> u.find_unit("ampere")
    ['ampere', 'amperes'] # partially correct, should also include 'A' and planck_current, similar to above
>>> u.find_unit('volt')
    ['volt', 'volts', 'electronvolt', 'electronvolts', 'planck_voltage'] # partially correct