provider/aws: Fix importing of EIP by IP address (#8970)
provider/aws: Fix importing of EIP by IP address (#8970)

* provider/aws: Fix importing of EIP by IP address

EIPs are meant to be imported by their allocation id, however, importing
by their EIP *appears* to work because this API actually accepts IP
lookup, despite the documentation asking for the allocation id.

This PR does:

- update docs on how to import EIPs
- fix case if user imported by IP, to switch to using the alloc id for
the resource id

I chose not to document that looking up by IP is a method of import,
because the AWS  API docs do not explicitly say that looking up by IP is
OK, so I'd rather people not do it if it's not documented to stay that
way.

Alternatively, we could parse the resource ID and reject it (remove from
state with error/warning) if it doesn't match the `eipalloc-*` format,
but I thought this was a bit better UX.

* fix issue with swapping IDs on EC2 Classic

* update docs

* update comment

nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws