loader: support imports from vendor directories

This fixes https://github.com/tinygo-org/tinygo/issues/1518.

loader/loader.go

@@ -53,6 +53,7 @@ type PackageJSON struct {
 
 	// Dependency information
 	Imports   []string
+	ImportMap map[string]string
 
 	// Error information
 	Error *struct {
@@ -411,6 +412,9 @@ func (p *Package) Import(to string) (*types.Package, error) {
 	if to == "unsafe" {
 		return types.Unsafe, nil
 	}
+	if newTo, ok := p.ImportMap[to]; ok && !strings.HasSuffix(newTo, ".test]") {
+		to = newTo
+	}
 	if imported, ok := p.program.Packages[to]; ok {
 		return imported.Pkg, nil
 	} else {