Go: Implement `stdin` models

[egregius313]

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Adds source models for the stdin threat model.

These models are implemented in QL instead of MaD due to limitations in MaD:

• MaD does not currently handle variable arguments as well as getAnImplicitVarargsArgument
• MaD has a way to access fields on structs, but not variables in a package. (This is generally not that important, but it is necessary for modeling os.Stdin).