cmd/go/internal/load: clone pgo variant's PackagePublic.Imports in split

Before this change the pgo and non-pgo variants Imports slices pointed
to the same array, so modifying the pgo variant's Imports slice to add
the .ForMain suffix modified the non-pgo vairant's Imports slice too.
This change clones the imports slice to avoid that.

Fixes #66218

Change-Id: Ic936086f2c31f2056988d6546216142e4fce4d8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/588275
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/go/internal/load/pkg.go

@@ -2943,8 +2943,10 @@ func setPGOProfilePath(pkgs []*Package) {
 					}
 					p1 := new(Package)
 					*p1 = *p
-					// Unalias the Internal.Imports slice, which is we're going to
+					// Unalias the Imports and Internal.Imports slices,
-					// modify. We don't copy other slices as we don't change them.
+					// which we're going to modify. We don't copy other slices as
+					// we don't change them.
+					p1.Imports = slices.Clone(p.Imports)
 					p1.Internal.Imports = slices.Clone(p.Internal.Imports)
 					p1.Internal.ForMain = pmain.ImportPath
 					visited[p] = p1

src/cmd/go/testdata/script/list_pgo_issue66218.txt

@@ -0,0 +1,28 @@
+# Test that pgo properly splits off the Imports field so that list doesn't alias
+# the non-pgo variant's slice when it modifies the pgo variant's Imports field to
+# add the [.ForMain] suffix.
+
+go list -f 'ImportPath: "{{.ImportPath}}", Imports: "{{.Imports}}", ImportMap: "{{.ImportMap}}"' m/a m/b
+cmp stdout want
+
+-- want --
+ImportPath: "m/a", Imports: "[m/b [m/a]]", ImportMap: "map[m/b:m/b [m/a]]"
+ImportPath: "m/b", Imports: "[m/c]", ImportMap: "map[]"
+-- go.mod --
+module m
+
+go 1.23
+
+-- a/a.go --
+package main
+
+import _ "m/b"
+-- a/default.pgo --
+-- b/b.go --
+package a
+
+import _ "m/c"
+
+-- c/c.go --
+package c
+