loader: fix testing a main package

This was broken because multiple packages in the program were named
'main', even one that was imported (by the generated main package).

This fixes tests for main packages.

loader/loader.go

@@ -343,12 +343,11 @@ func (p *Package) Check() error {
 	checker.Importer = p
 
 	packageName := p.ImportPath
-	if p.Name == "main" {
-		// The main package normally has a different import path, such as
-		// "command-line-arguments" or "./testdata/cgo". Therefore, use the name
-		// "main" in such a case: this package isn't imported from anywhere.
-		// This is safe as it isn't possible to import a package with the name
-		// "main".
+	if p == p.program.MainPkg() {
+		if p.Name != "main" {
+			// Sanity check. Should not ever trigger.
+			panic("expected main package to have name 'main'")
+		}
 		packageName = "main"
 	}
 	typesPkg, err := checker.Check(packageName, p.program.fset, p.Files, &p.info)