fix: isSysPkg inference logic (#66) and Writer bugs (#61)

[angrychow]

What type of PR is this?

Fixes #61 , #66

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fix issue:

1. Go Parser: Helper function isSysPkg incorrectly identifies local packages as system packages
2. go writer: bad output because of test packages. no output for non code files

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修复了：

1. Go Parser: Helper function isSysPkg incorrectly identifies local packages as system packages
2. go writer: bad output because of test packages. no output for non code files

[angrychow]

I have tested the build.Import function, this function is rely on file system, and having a cache would be useful if there are massive isSysPkg queries.