compiler: read embedcfg files, parse go:embed directives

compiler: read embedcfg files, parse go:embed directives

This change reads go:embed directives and attaches them to variables.
We still don't do anything with the directives.

This change also reads the file passed in the -fgo-embedcfg option.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/281533