Refactor getScript logic to a single file.

[lijunle]

• Refactor getScript logic to a single file.
• Apply cache to improve performance.
• APIs are not affected.
• Some invalid failing test cases are resolved.

[codecov-io]

Current coverage is 97.50%

Merging #97 into master will decrease coverage by -0.75% as of 2da2f78

@@            master    #97   diff @@
=====================================
  Files           15     16     +1
  Stmts          343    360    +17
  Branches        79     83     +4
  Methods          0      0       
=====================================
+ Hit            337    351    +14
- Partial          4      6     +2
- Missed           2      3     +1

Review entire Coverage Diff as of 2da2f78

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