fix(package dictionary): removed package_key_dict since it is redundant and can cause errors

[spaulins-usgs]

package_key_dict has been removed and replaced with a property getter that gives a deprecation warning and returns the package_type_dict, which is functionally similar.

[codecov]

Codecov Report

Merging #1690 (1d1fe8a) into develop (24448b7) will decrease coverage by 1.9%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           develop   #1690     +/-   ##
=========================================
- Coverage     71.8%   70.0%   -1.9%     
=========================================
  Files          253     253             
  Lines        55884   54524   -1360     
=========================================
- Hits         40177   38202   -1975     
- Misses       15707   16322    +615     

Impacted Files	Coverage Δ
flopy/mf6/mfbase.py	73.2% <100.0%> (-8.0%)	⬇️
flopy/mf6/modflow/mfgwtgwt.py	33.3% <0.0%> (-29.8%)	⬇️
flopy/mf6/modflow/mfgwfcsub.py	39.0% <0.0%> (-28.2%)	⬇️
flopy/utils/compare.py	2.9% <0.0%> (-27.5%)	⬇️
flopy/mf6/modflow/mfgwtlkt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtmwt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtsft.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtuzt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtcnc.py	46.1% <0.0%> (-23.5%)	⬇️
flopy/mf6/modflow/mfgwtsrc.py	46.1% <0.0%> (-23.5%)	⬇️
... and 88 more