go/loader: improve robustness in face of input errors

    Before this change, many kinds of error would cause the loader to stop.
    making it brittle when analyzing large codebases, as in "godoc -analysis".

    This change moves operations that used to occur during
    configuration---(*build.Context).Import, loading, and parsing of
    initial packages---into the Load call, and ensures that all failures
    during Loading are reported at the end so that the maximum amount of
    progress is made.

    Also: redesign the tests and add many new cases.

Change-Id: Ia8cd99416af7c5d4a5fe133908adfa83676d401f
Reviewed-on: https://go-review.googlesource.com/3626
Reviewed-by: Robert Griesemer <[email protected]>

cmd/eg/eg.go

@@ -61,9 +61,7 @@ func doMain() error {
 	}
 
 	// The first Created package is the template.
-	if err := conf.CreateFromFilenames("template", *templateFlag); err != nil {
-		return err //  e.g. "foo.go:1: syntax error"
-	}
+	conf.CreateFromFilenames("template", *templateFlag)
 
 	if len(args) == 0 {
 		fmt.Fprint(os.Stderr, usage)

cmd/ssadump/main.go

@@ -203,7 +203,7 @@ func doMain() error {
 		}
 
 		if runtime.GOARCH != build.Default.GOARCH {
-			return fmt.Errorf("cross-interpretation is not yet supported (target has GOARCH %s, interpreter has %s)",
+			return fmt.Errorf("cross-interpretation is not supported (target has GOARCH %s, interpreter has %s)",
 				build.Default.GOARCH, runtime.GOARCH)
 		}