"ignore" build tag is special in module mode

[segevfiner]

See golang/go#29598

The README shows using an "assets_generate.go" with the "ignore" build tag, but that is ignored for the purposes of modules which will make it not pin the version of vfsgen if there isn't another file importing it.

Unrelated to this I had trouble getting this to work with modules, the importPathToDir trick doesn't seem to work in modules mode. So I had to move the go:generate comment to the main module and use the Filename options to vfsgen to get it to write the file to the right place. This package could really use better/updated docs.

[dmitshur]

Thanks for reporting the problem with ignore build tag, that indeed needs to be fixed.

Can you please open a separate issue about the problem with importPathToDir? It does need to be run inside the main module, but otherwise should work. It’ll be easier to discuss it in more detail in a dedicated issue. Thanks.