Prevent declared field to have additional lookups

[akspi]

I've included an extra check for all fields other than the LookupChoiceFilter to check that if a field which is specified in Meta.fields also exists in declared fields, then make sure that the lookup expressions match; ie declaration of fields in both Meta.fields and the declare fields agree on the same lookup expressions for that field.

I'm not sure if this covers all possible cases as I'm not entirely familiar with the codebase yet. Any feedback would be greatly appreciated! #1013

[codecov-io]

Codecov Report

Merging #1019 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1019   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files          15       15           
  Lines        1198     1198           
=======================================
  Hits         1178     1178           
  Misses         20       20

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <100%> (ø)	⬆️

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[akspi]

@rpkilby Was wondering if this was still required?

[rpkilby]

Hi @PaiAkshay998. You can close this. The issue isn't directly related to LookupChoiceFilter (although it happens to be used in the original SO question). I've opened #1061, which fixes this more generally.