compiler: rename import path if it lies in TINYGOPATH

This allows importing (for example) both
"github.com/tinygo-org/tinygo/src/machine" and "machine" without issues.
The former is renamed to just "machine".

loader/loader.go

@@ -16,16 +16,16 @@ import (
 
 // Program holds all packages and some metadata about the program as a whole.
 type Program struct {
-	Build         *build.Context
-	OverlayBuild  *build.Context
-	ShouldOverlay func(path string) bool
-	Packages      map[string]*Package
-	sorted        []*Package
-	fset          *token.FileSet
-	TypeChecker   types.Config
-	Dir           string // current working directory (for error reporting)
-	TINYGOROOT    string // root of the TinyGo installation or root of the source code
-	CFlags        []string
+	Build        *build.Context
+	OverlayBuild *build.Context
+	OverlayPath  func(path string) string
+	Packages     map[string]*Package
+	sorted       []*Package
+	fset         *token.FileSet
+	TypeChecker  types.Config
+	Dir          string // current working directory (for error reporting)
+	TINYGOROOT   string // root of the TinyGo installation or root of the source code
+	CFlags       []string
 }
 
 // Package holds a loaded package, its imports, and its parsed files.
@@ -48,8 +48,9 @@ func (p *Program) Import(path, srcDir string) (*Package, error) {
 
 	// Load this package.
 	ctx := p.Build
-	if p.ShouldOverlay(path) {
+	if newPath := p.OverlayPath(path); newPath != "" {
 		ctx = p.OverlayBuild
+		path = newPath
 	}
 	buildPkg, err := ctx.Import(path, srcDir, build.ImportComment)
 	if err != nil {