fix:简化dynamic内load函数

[HaiTing-Zhu]

Checklist

• [ ] npm test passes
• [ ] tests are included
• [ ] documentation is changed or added
• [ ] commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/dvajs/dva/ISSUE_URL

[codecov-io]

Codecov Report

Merging #2386 into master will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #2386   +/-   ##
=======================================
  Coverage   85.88%   85.88%           
=======================================
  Files          18       18           
  Lines         517      517           
  Branches      114      116    +2     
=======================================
  Hits          444      444           
  Misses         59       59           
  Partials       14       14           

Impacted Files	Coverage Δ
packages/dva/src/dynamic.js	6.00% <0.00%> (ø)

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[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[sorrycc]

code style 的修改就不单独合了。