generated models across loaded packages causes error merging type systems

[StevenACoffman]

What happened?

If models are generated in a package that has already been loaded, and
that package refers to another package that has already been loaded, we
can find ourselves in a position where it appears that a GQL union is
not satisfied.

#1020 introduces the
code that fails in this scenario. This was supposed to be addressed in #1491 by @wendorf
but per @ipfans #1491 (comment) it appears to still be occurring.

This repository currently reproduces the issue reliably:
https://github.com/ipfans/gqlgen-error

and is currently getting this error:

merging type systems failed: unable to bind to interface: xxx/src/business/entity.A does not satisfy the interface gitlab.com/hookactions/gqlgen-relay/relay.Node

[ipfans]

I am trying to debug this bug, then I found cfg.ReloadAllPackages() is never called before error. After add this, I think everything works now. I will submit a PR soon.

[frederikhors]

Should we release a patch version with all these news?

[StevenACoffman]

@frederikhors I released v0.15.0 right after this merged as this was the final bug fix I was waiting for to land. We're using this same version in production at scale at Khan, and I feel like it's been very solid.

[frederikhors]

Yeah it is solid. gqlgen is amazing. Really makes me thankful for life.

Can we open the discussions tab, @StevenACoffman, please, so we can be able to move there issues that are simple questions? (already requested in #1722)