go/packages: go list -deps=false if no NeedDeps

[jirfag]

Don't load deps by go list if NeedDeps wasn't set. Before that
deps were loading if NeedImports was set.

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Message from Denis Isaev:

Patch Set 1:

I've made the pull request with reverted in https://go-review.googlesource.com/c/tools/+/186337/ changes

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This PR (HEAD: 016959b) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/194837 to see it.

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Message from Michael Matloob:

Patch Set 2:

(2 comments)

Sorry for the slowness to review. I'll have some more comments, but can you sync first? I want to make sure that the CL still makes sense at tip.

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