placing non-go files in subdirs prevents `go mod vendor` from working correctly

[codyps]

See https://github.com/golang/go/issues/26366 for some backstory here.

Essentially, go (and especially go mod vendor) doesn't like having subdirectories that aren't go packages that are depended on. Currently, this package tracks include/header files and binary library objects in subdirectories. As a result, any package that depends on bls-eth-go-binary (at any distance) will be broken when used with go mod vendor, because go mod vendor will not keep the header & library files because it doesn't consider them to be members of the module being depended on.

Potential fixes:

• create an empty.go file in each subdirectory and depend on them so go sees the subdirectories as modules that need to be downloaded
• flatten the subdirectories and include all files in an existing go module directory. ie: name files bls/lib-linux-arm64-libbls384_256.a, etc. (as bls is the directory of the go module that links these libraries).

[herumi]

If I place an empty.go file in each directory, can I keep the original file names? Since this project is sometimes used by non-Go projects, I'd like to minimize changes to file names and directory structure as much as possible.