go/build: fix ImportDir to report PkgTarget for directories in GOROOT/GOPATH

This makes ImportDir("$GOROOT/src/math", 0)
and Import("math", "", 0) equivalent. It was an
oversight that they were not before.

An upcoming change to the go command relies on
the two returning the same results.

Change-Id: I187da4830fae85f8dde673c22836ff2da6801047
Reviewed-on: https://go-review.googlesource.com/75290
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/go/build/build.go

@@ -544,6 +544,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				p.Goroot = true
 				p.ImportPath = sub
 				p.Root = ctxt.GOROOT
+				setPkga() // p.ImportPath changed
 				goto Found
 			}
 		}
@@ -571,6 +572,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				// Record it.
 				p.ImportPath = sub
 				p.Root = root
+				setPkga() // p.ImportPath changed
 				goto Found
 			}
 		}

src/go/build/build_test.go

@@ -382,3 +382,16 @@ func TestImportVendorParentFailure(t *testing.T) {
 		t.Fatalf("error on failed import does not mention GOROOT/src/vendor directory:\n%s", e)
 	}
 }
+
+func TestImportDirTarget(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+	ctxt := Default
+	ctxt.GOPATH = ""
+	p, err := ctxt.ImportDir(filepath.Join(ctxt.GOROOT, "src/path"), 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.PkgTargetRoot == "" || p.PkgObj == "" {
+		t.Errorf("p.PkgTargetRoot == %q, p.PkgObj == %q, want non-empty", p.PkgTargetRoot, p.PkgObj)
+	}
+}