Use meta.GetExternalName(mg) to check whether the managed resource exists

[zzxwill]

Description of your changes

If we don't use meta.GetExternalName(mg) as the identifier when the managed resource exists, or there will be more than one external cloud resources are created.

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested