loader: don't panic when main package is not named 'main'

This can in fact happen in practice, so return an actual error message
instead.

errors_test.go

@@ -24,6 +24,7 @@ func TestErrors(t *testing.T) {
 		{name: "cgo"},
 		{name: "compiler"},
 		{name: "interp"},
+		{name: "invalidmain"},
 		{name: "linker-flashoverflow", target: "cortex-m-qemu"},
 		{name: "linker-ramoverflow", target: "cortex-m-qemu"},
 		{name: "linker-undefined", target: "darwin/arm64"},

loader/loader.go

@@ -418,8 +418,12 @@ func (p *Package) Check() error {
 	packageName := p.ImportPath
 	if p == p.program.MainPkg() {
 		if p.Name != "main" {
-			// Sanity check. Should not ever trigger.
+			return Errors{p, []error{
-			panic("expected main package to have name 'main'")
+				scanner.Error{
+					Pos: p.program.fset.Position(p.Files[0].Name.Pos()),
+					Msg: fmt.Sprintf("expected main package to have name \"main\", not %#v", p.Name),
+				},
+			}}
 		}
 		packageName = "main"
 	}

testdata/errors/invalidmain.go

@@ -0,0 +1,6 @@
+// some comment to move the first line
+
+package foobar
+
+// ERROR: # command-line-arguments
+// ERROR: invalidmain.go:3:9: expected main package to have name "main", not "foobar"