Allow go modules in analysis

[romaindoumenc]

As mentioned in golang/go#37054 , the current analyzer forces a GOPATH environment set-up.
Changing this to default to modules (while still allowing environment variable to revert to GOPATH is needed).

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[romaindoumenc]

Closing as this is a partial change – so it probably won’t do. Doing a full version first.

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