What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:

Avoid expensive per-target calls out to goimports from gengo.

This improve codegen update and verify times by about 25%.

Special notes for your reviewer:

This is based on f63343f from @thockin.

This depends on a gengo/v2 change: kubernetes/gengo#277

The main changes here are to fix sloppy import handling that was masked by gengo's import fixup, disable gengo's import fix up with kubernetes/gengo#277, and then generate with the import name changes that result.

The main import changes are:

• Before, conversion-gen would import packages under the same package alias as the local package. For example, the conversion v1 package would import a types package using the v1 package alias. This is legal go code, but is less clear than an import like corev1 or metav1, so we've added a rule that imported type aliases must not match the local package alias exactly.
• Before, generated code like "fmt.Errorf" could be used without importing "fmt" and goimports would fix up the imports by adding "fmt". Now, our gengo adds all imports and follows the gengo convention of redundantly defining the import as fmt "fmt". We could further change gengo to avoid redundant import aliases, but for this change we're just switching the codegen to be consistent for all types, using gengo's conventions, instead of a mix or gengo and goimport conventions.

Does this PR introduce a user-facing change?

NONE

/sig api-machinery
/assign @thockin @skitt