cmd/cue: correctly extract transitive dependencies in 'get go'

The refactor in https://cuelang.org/cl/1173100 fixed multiple bugs
but unfortunately introduced a tricky hidden bug.
Rather than extracting all used packages to cover all dependencies,
we were repeatedly extracting the first package instead.

'cue get go' now extracts all dependencies as expected both with
and without the --local flag.

Fixes #3033.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Idc707758f758736a550a851f9bcc55b59f4b704b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193699
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/get_go.go

@@ -565,7 +565,7 @@ func (e *extractor) extractPkg(root string, p *packages.Package) error {
 	for path := range e.usedPkgs {
 		if !e.done[path] {
 			e.done[path] = true
-			if err := e.extractPkg(root, e.allPkgs[p.PkgPath]); err != nil {
+			if err := e.extractPkg(root, e.allPkgs[path]); err != nil {
 				return err
 			}
 		}

cmd/cue/cmd/testdata/script/get_go_non_local.txtar

@@ -17,11 +17,9 @@ cmp go.mod go.mod.golden
 
 # Verify that all transitive dependencies are generated properly as well.
 # If any are missing, the evaluation would fail to load them.
-# TODO: broken due to https://cuelang.org/issue/3033.
 go get externalmod.test/imports/root@v1.0.0
 exec cue get go externalmod.test/imports/root
-! exec cue eval uses_imports_root.cue
-stderr 'cannot find package "externalmod.test/imports/sub1"'
+exec cue eval uses_imports_root.cue
 
 -- cue.mod/module.cue --
 module: "mod.test/local"

cmd/cue/cmd/testdata/script/get_go_vendor.txtar

@@ -5,9 +5,7 @@ cmp x/x_go_gen.cue x_go_gen.cue.golden
 
 # Verify that all transitive dependencies were generated properly.
 # If any are missing, the evaluation would fail to load them.
-# TODO: broken due to https://cuelang.org/issue/3033.
-! exec cue eval ./x
-stderr 'no CUE files in mod.test/z'
+exec cue eval ./x
 
 -- go.mod --
 module mod.test