loader: improve error messages for failed imports

Add location information (whenever possible) to failed imports. This
helps in debugging where an incorrect import came from.

For example, show the following error message:

    /home/ayke/src/github.com/tinygo-org/tinygo/src/machine/machine.go:5:8: cannot find package "foobar" in any of:
        /usr/local/go/src/foobar (from $GOROOT)
        /home/ayke/src/foobar (from $GOPATH)

Instead of the following:

    error: cannot find package "foobar" in any of:
        /usr/local/go/src/foobar (from $GOROOT)
        /home/ayke/src/foobar (from $GOPATH)

compiler/compiler.go

@@ -251,13 +251,17 @@ func (c *Compiler) Compile(mainPath string) []error {
 			return []error{err}
 		}
 	} else {
-		_, err = lprogram.Import(mainPath, wd)
+		_, err = lprogram.Import(mainPath, wd, token.Position{
+			Filename: "build command-line-arguments",
+		})
 		if err != nil {
 			return []error{err}
 		}
 	}
 
-	_, err = lprogram.Import("runtime", "")
+	_, err = lprogram.Import("runtime", "", token.Position{
+		Filename: "build default import",
+	})
 	if err != nil {
 		return []error{err}
 	}