cmd/go: revert "cmd/go: no longer eval symlinks in inDir"

It breaks on the Darwin builders, with

--- FAIL: TestTestCacheInputs (21.65s)
    go_test.go:5371: file.txt: old=2018-08-02 15:12:21.136438 -0700 PDT m=+219.458659208, info.ModTime=2018-08-02 15:12:21 -0700 PDT
    go_test.go:5377: running testgo [test testcache]
    go_test.go:5377: standard output:
    go_test.go:5377: ok  	testcache	0.013s

    go_test.go:5378: running testgo [test testcache]
    go_test.go:5378: standard output:
    go_test.go:5378: ok  	testcache	(cached)

    go_test.go:5382: running testgo [test testcache]
    go_test.go:5382: standard output:
    go_test.go:5382: ok  	testcache	0.172s

    go_test.go:5384: running testgo [test testcache]
    go_test.go:5384: standard output:
    go_test.go:5384: ok  	testcache	(cached)

    go_test.go:5387: running testgo [test testcache -run=FileSize]
    go_test.go:5387: standard output:
    go_test.go:5387: ok  	testcache	0.012s

    go_test.go:5388: running testgo [test testcache -run=FileSize]
    go_test.go:5388: standard output:
    go_test.go:5388: ok  	testcache	(cached)

    go_test.go:5391: running testgo [test testcache -run=FileSize]
    go_test.go:5391: standard output:
    go_test.go:5391: ok  	testcache	0.008s

    go_test.go:5393: running testgo [test testcache -run=FileSize]
    go_test.go:5393: standard output:
    go_test.go:5393: ok  	testcache	(cached)

    go_test.go:5396: running testgo [test testcache -run=Chdir]
    go_test.go:5396: standard output:
    go_test.go:5396: ok  	testcache	0.417s

    go_test.go:5397: running testgo [test testcache -run=Chdir]
    go_test.go:5397: standard output:
    go_test.go:5397: ok  	testcache	(cached)

    go_test.go:5400: running testgo [test testcache -run=Chdir]
    go_test.go:5400: standard output:
    go_test.go:5400: ok  	testcache	(cached)

    go_test.go:5401: did not notice file size change
    go_test.go:5401: pattern \(cached\) found unexpectedly in standard output
FAIL

Original CL description:

    Evaluating the symlinks was slowing down test cache checks.

    Fixes #26562
    Fixes #26726

            ijt:~/gopath/src/issue26562$ cat foo_test.go
            package foo_test

            import (
                    "fmt"
                    "os"
                    "path/filepath"
                    "testing"
            )

            // package and imports snipped
            func TestCache(t *testing.T) {
                    tmp := os.TempDir()
                    for i := 0; i < 1000000; i++ {
                            os.Stat(filepath.Join(tmp, fmt.Sprintf("%d", i)))
                    }
            }
            ijt:~/gopath/src/issue26562$ time ~/github/go/bin/go test -count=1
            PASS
            ok      issue26562      9.444s

            real    0m10.021s
            user    0m2.344s
            sys     0m7.835s
            ijt:~/gopath/src/issue26562$ time ~/github/go/bin/go test .
            ok      issue26562      (cached)

            real    0m0.802s
            user    0m0.551s
            sys     0m0.306s

Updates #26562
Updates #26726

Change-Id: I7914ee57dc75bcbd7f0ea01c70bed97d67c810ea
Reviewed-on: https://go-review.googlesource.com/127715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/test/test.go

@@ -1448,7 +1448,15 @@ func computeTestInputsID(a *work.Action, testlog []byte) (cache.ActionID, error)
 }
 
 func inDir(path, dir string) bool {
-	return str.HasFilePathPrefix(path, dir)
+	if str.HasFilePathPrefix(path, dir) {
+		return true
+	}
+	xpath, err1 := filepath.EvalSymlinks(path)
+	xdir, err2 := filepath.EvalSymlinks(dir)
+	if err1 == nil && err2 == nil && str.HasFilePathPrefix(xpath, xdir) {
+		return true
+	}
+	return false
 }
 
 func hashGetenv(name string) cache.ActionID {