fix imports for splitdns

[keeleysam]

What this PR does / why we need it:
Currently if you use import on tailscale_dns_split_nameservers, the domain attribute will get set to null in the terraform state, and thus will result in

Since the resource ID and domain are the same, set domain to the resource ID.
sample code:

import {
  to = tailscale_dns_split_nameservers.example-com
  id = "example.com"
}

resource "tailscale_dns_split_nameservers" "example-com" {
  domain      = "example.com"
  nameservers = [
        "1.2.3.4",
        "5.6.7.8"
  ]
}

Before:

# tailscale_dns_split_nameservers.example-com will be updated in-place
~ resource "tailscale_dns_split_nameservers" "example-com" {
    + domain      = "example.com"
      id          = "example.com"
      nameservers = [
        "1.2.3.4",
        "5.6.7.8"
      ]
  }

after:

  # tailscale_dns_split_nameservers.example-com will be imported
resource "tailscale_dns_split_nameservers" "example-com" {
    domain      = "example.com"
    id          = "example.com"
    nameservers = [
        "1.2.3.4",
        "5.6.7.8"
      ]
}

Which issue this PR fixes (use fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):

see above

Special notes for your reviewer:

[mpminardi]

Thank you for catching and fixing this!