x/tools/go/packages: lists PGO variants of the unsafe package

[findleyr]

When loading the cmd module, go/packages synthesizes an intermediate test variantpgo variant for the 'unsafe' package. This doesn't make sense, since the unsafe package has no imports. Intermediate test variants exist to remap imports.

> gopackages -deps cmd/... | grep "unsafe "
Go package "unsafe [cmd/compile]":
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"
	import "unsafe" => "unsafe [cmd/compile]"

CC @matloob @adonovan @bcmills

[bcmills]

Those aren't test variants — those are PGO variants (attn @cherrymui @prattmic).

[findleyr]

I see. I feel like go/packages should probably filter these out? This could be problematic for many tools.

[findleyr]

Per discussion, I think we should set -pgo=off by default (especially in light of the performance cost, e.g. #60455). If someone wants to file a proposal to add a NeedPGO flag, they can, but I don't see a reason why users of go/packages would care about PGO variants.

CC @dominikh

[gopherbot]

Change https://go.dev/cl/498557 mentions this issue: go/packages: pass -pgo=off on go1.21 and later