cmd/go: do not try to load 'all' packages with invalid import paths

Before this change, when we tried to compute the set of packages in
'all', we'd add packages with invalid import paths to the set and try to
load them, which would fail. Instead, do not add them to the list of
packages to load in the second iteration of the loader. We'll still
return errors for invalid imports in the importing packages.
Change-Id: I682229011f555ed1d0c827f79100c1c43bf7f93a
Reviewed-on: https://go-review.googlesource.com/c/go/+/673655
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/modload/load.go

@@ -2009,6 +2009,13 @@ func (ld *loader) stdVendor(parentPath, path string) string {
 // starting with a list of the import paths for the packages in the main module.
 func (ld *loader) computePatternAll() (all []string) {
 	for _, pkg := range ld.pkgs {
+		if module.CheckImportPath(pkg.path) != nil {
+			// Don't add packages with invalid paths. This means that
+			// we don't try to load invalid imports of the main modules'
+			// packages. We will still report an errors invalid imports
+			// when we load the importing package.
+			continue
+		}
 		if pkg.flags.has(pkgInAll) && !pkg.isTest() {
 			all = append(all, pkg.path)
 		}

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

@@ -0,0 +1,17 @@
+! go list all
+! stderr 'panic'
+stderr 'invalid import path'
+
+# go list produces a package for 'p' but not for ''
+go list -e all
+cmp stdout wantlist.txt
+-- wantlist.txt --
+example.com/e
+-- go.mod --
+module example.com/e
+
+go 1.25
+-- p.go --
+package p
+
+import ""