Virtual site parameter lookups should return multiple (or no?) parameters if multiple have the same SMIRKS

[mattwthompson]

Describe the bug

This is a follow-on to #1847 because virtual site parameters are different.

To Reproduce

from openff.toolkit import ForceField

force_field = ForceField("""<?xml version="1.0" encoding="utf-8"?>
<SMIRNOFF version="0.3" aromaticity_model="OEAroModel_MDL">
    <Electrostatics version="0.4" scale12="0.0" scale13="0.0" scale14="0.8333333333" scale15="1.0" cutoff="9.0 * angstrom" switch_width="0.0 * angstrom" periodic_potential="Ewald3D-ConductingBoundary" nonperiodic_potential="Coulomb" exception_potential="Coulomb"></Electrostatics>
    <VirtualSites version="0.3" exclusion_policy="parents">
        <VirtualSite smirks="[#7:1](-[*:2])(-[*:3])-[*:4]"  type="TrivalentLonePair" match="once" distance="-0.5 * angstrom"  charge_increment1="1.0 * elementary_charge" charge_increment2="0.0 * elementary_charge" charge_increment3="0.0 * elementary_charge" charge_increment4="0.0 * elementary_charge" rmin_half="0.0 * angstrom" epsilon="0.0 * kilocalorie_per_mole" name="EP1"></VirtualSite>
        <VirtualSite smirks="[#7:1](-[*:2])(-[*:3])-[*:4]"  type="TrivalentLonePair" match="once" distance="2.0 * angstrom"  charge_increment1="-5.0 * elementary_charge" charge_increment2="0.0 * elementary_charge" charge_increment3="0.0 * elementary_charge" charge_increment4="0.0 * elementary_charge" rmin_half="0.0 * angstrom" epsilon="0.0 * kilocalorie_per_mole" name="EP2"></VirtualSite>
    </VirtualSites>
</SMIRNOFF>""")

print(force_field["VirtualSites"]["[#7:1](-[*:2])(-[*:3])-[*:4]"].distance)
# 2.0 angstrom

Output

The 2 A parameter is returned, but both parameters really ought to be returned. This example is a bit stripped-down and funky for the sake of complexity, but there are valid use cases for multiple virtual site parameters having the same SMIRKS and different distances/charges/other parameters. (Compare this to i.e. vdW or torsion interactions, which should only get "one" parameter at a time.)

Additional context

I am working around this downstream, looking up parameters by the criteria I believe define uniqueness: https://github.com/openforcefield/openff-interchange/blob/v0.3.25/openff/interchange/components/toolkit.py#L200-L206

I think having SMIRKS-only lookups is more of a hassle than it's worth and would be fine to get __getitem__ not implemented for this case. It could be looked up with a tuple of SMIRKS, name, and possibly match, but that's enough trouble to mitigate the benefit of syntactic sugar. There should be a well-defined set of behaviors in the proper API for this, something around .get_parameter.

There is some discussion in the SMIRNOFF spec but some important details are yet unresolved: openforcefield/standards#64

[mattwthompson]

I think #1861 (0.16.1?) resolves this, please re-open if that is not the case