Fix incorrect Protobuf FileDescriptor references

[Mrmann87]

Fixes several well known types that were referencing incorrect descriptors.
Additionally, changes the field behavior and source context imports,
which are part of the standard protobuf distribution, to google.golang.org/protobuf
instead of google.golang.org/genproto.

[PrasanthV454]

Ran tests on these fields to verify. LGTM.

[milindl]

Looks good to me. I've created a PR on an internal branch for the CI to run, and I'll add a changelog entry. #989

[milindl]

Closed with merge to master of #989, thanks for this @Mrmann87 !

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❌ PrasanthV454
❌ Mrmann87
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