support as on import statement

[koxudaxi]

This PR supports as on import statement.

Related Issues

#102

[codecov]

Codecov Report

Merging #112 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #112   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          853       865   +12     
  Branches       175       178    +3     
=========================================
+ Hits           853       865   +12     

Impacted Files	Coverage Δ
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/parser/base.py	100.00% <100.00%> (ø)

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