Update how standard primitives are imported internally

[kmax12]

This PR updates how the standard primitives are imported to better support loading extra primitives from entry points.

As things stand, if a primitive is loaded from an entry point it overwrites the standard primitive.

With this PR, a user can still access the standard primitives using from featuretools.primitives.standard import PrimitiveName

[codecov]

Codecov Report

Merging #482 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #482   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         100      102    +2     
  Lines        8607     8607           
=======================================
  Hits         8283     8283           
  Misses        324      324

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	96.73% <100%> (-0.02%)	⬇️
featuretools/primitives/api.py	100% <100%> (ø)	⬆️
featuretools/primitives/standard/api.py	100% <100%> (ø)
featuretools/primitives/standard/__init__.py	100% <100%> (ø)

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[rwedge]

Usually we use an api.py file to manage the individual imports for a module and have __init__.py import everything from api.py.

[rwedge]

Looks good