[release-branch.go1.12] cmd/go/internal/imports: use the full path to resolve symlinks

info.Name returns a name relative to the directory, so we need to
prefix that directory in the Stat call.

(This was missed in CL 141097 due to the fact that the test only
happened to check symlinks in the current directory.)

This allows the misc/ tests to work in module mode on platforms that
support symlinks.

Updates #30228
Updates #28107
Fixes #31763

Change-Id: Ie31836382df0cbd7d203b7a8b637c4743d68b6f3
Reviewed-on: https://go-review.googlesource.com/c/163517
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/175441Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/imports/scan.go

@@ -26,7 +26,7 @@ func ScanDir(dir string, tags map[string]bool) ([]string, []string, error) {
 		// If the directory entry is a symlink, stat it to obtain the info for the
 		// link target instead of the link itself.
 		if info.Mode()&os.ModeSymlink != 0 {
-			info, err = os.Stat(name)
+			info, err = os.Stat(filepath.Join(dir, name))
 			if err != nil {
 				continue // Ignore broken symlinks.
 			}

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

@@ -2,16 +2,31 @@ env GO111MODULE=on
 [!symlink] skip
 
 # 'go list' should resolve modules of imported packages.
-go list -deps -f '{{.Module}}'
+go list -deps -f '{{.Module}}' .
 stdout golang.org/x/text
 
-# They should continue to resolve if the importing file is a symlink.
+go list -deps -f '{{.Module}}' ./subpkg
+stdout golang.org/x/text
+
+# Create a copy of the module using symlinks in src/links.
 mkdir links
+symlink links/go.mod -> $GOPATH/src/go.mod
+symlink links/issue.go -> $GOPATH/src/issue.go
+mkdir links/subpkg
+symlink links/subpkg/issue.go -> $GOPATH/src/subpkg/issue.go
+
+# We should see the copy as a valid module root.
 cd links
-symlink go.mod -> ../go.mod
-symlink issue.go -> ../issue.go
+go env GOMOD
+stdout links[/\\]go.mod
+go list -m
+stdout golang.org/issue/28107
 
-go list -deps -f '{{.Module}}'
+# The symlink-based copy should contain the same packages
+# and have the same dependencies as the original.
+go list -deps -f '{{.Module}}' .
+stdout golang.org/x/text
+go list -deps -f '{{.Module}}' ./subpkg
 stdout golang.org/x/text
 
 -- go.mod --
@@ -21,3 +36,7 @@ module golang.org/issue/28107
 package issue
 
 import _ "golang.org/x/text/language"
+-- subpkg/issue.go --
+package issue
+
+import _ "golang.org/x/text/language"