`InsertPreviousObjects` assumes APIs have already been generated

### What happened?

I'm prototyping a change to upjet that'd have it generate namespaced API types and controllers. See https://github.com/crossplane/upjet/pull/470.

I'm testing my change with provider-upjet-aws. Part of that involves generating a completely new `apis-namespaced` directory. When I run `make generate` upjet panics when calling `versionGen.InsertPreviousObjects`:

```
panic: cannot insert type definitions from the previous versions into the package scope for group "lakeformation.aws.upbound.io": cannot load the previous versions of "aws_lakeformation_permissions" from path github.com/upbound/provider-aws/apis-namespaced/lakeformation/v1beta1: err: chdir /home/negz/control/crossplane-contrib/provider-upjet-aws/apis-namespaced/lakeformation/v1beta1: no such file or directory: stderr:
```

I've spent all day staring at the `versionGen.InsertPreviousObjects` (introduced in https://github.com/crossplane/upjet/pull/402) but I'm having a lot of trouble understanding what it does. It seems to me that the implementation doesn't match its Godoc or the description in https://github.com/crossplane/upjet/pull/402.

I _think_ there's an order dependency issue:

1. When generating API Go types for (e.g.) guardduty v1beta1
2. If any guardduty resources have v1beta1 as a previous version
3. Then try to load types from the `v1beta1/zz_*_types.go` files for all guardduty resources into package scope

However at the time `InsertPreviousObjects` is called there's no guarantee that `v1beta1/zz_*_types.go` files exist for all guardduty resources. In fact the first time code generation runs, they definitely won't exist.

### How can we reproduce it?

In provider-upjet-aws delete all the generated files (and their directories, but not e.g. `generate.go`) under `apis/` and run `make generate`.