loader: merge roots from both Go and TinyGo in a cached directory

This commit changes the way that packages are looked up. Instead of
working around the loader package by modifying the GOROOT variable for
specific packages, create a new GOROOT using symlinks. This GOROOT is
cached for the specified configuration (Go version, underlying GOROOT
path, TinyGo path, whether to override the syscall package).

This will also enable go module support in the future.

loader/loader.go

@@ -22,8 +22,6 @@ import (
 type Program struct {
 	mainPkg      string
 	Build        *build.Context
-	OverlayBuild *build.Context
-	OverlayPath  func(path string) string
 	Packages     map[string]*Package
 	sorted       []*Package
 	fset         *token.FileSet
@@ -54,10 +52,6 @@ func (p *Program) Import(path, srcDir string, pos token.Position) (*Package, err
 
 	// Load this package.
 	ctx := p.Build
-	if newPath := p.OverlayPath(path); newPath != "" {
-		ctx = p.OverlayBuild
-		path = newPath
-	}
 	buildPkg, err := ctx.Import(path, srcDir, build.ImportComment)
 	if err != nil {
 		return nil, scanner.Error{