embed, cmd/go: embedded files add support for fs.Open

[hktalent]

embed, cmd/go: embedded files add support for fs.Open
That is to say, once I embed a directory, reading and writing to this directory can be operated using the regular file api
Of course, you can set a policy when embedding, for example
Use local files first, then embedded files

[mengzhuo]

If you wish to change API please take a look at proposal wiki and reform your current issue, thanks.