compiler: open byte slice and string embeds using the absolute path

compiler: open byte slice and string embeds using the absolute path

The paths vector contains the names of the files that the embed_files_
map is keyed by. While the code processing embed.FS values looks up
the paths in the embed_files_ map, the code processing string and byte
slice embeds tries opening the files using their names directly. Look
up the full paths in the embed_files_ map when opening them.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/291429