This is step 3 of 6 in a multi-stage migration
to move the well-known types
from the google.golang.org/genproto module
to the google.golang.org/protobuf module.

The generated Go packages for field_mask.proto, api.proto,
type.proto, and source_context.proto are being moved over
to this module alongside all the other well-known types.

In order to move these types between two modules,
there needs to be a sequence of changes submitted in
decently rapid succession. It is impossible to atomically
make these changes, so a brief breakage is inevitable.
The steps are as follows:

Step 1: Submit a change to cloud.google.com/go/internal/gapicgen
to avoid generating the well-known types. Otherwise, the tool
will undo the changes made in step 3.
See https://code-review.googlesource.com/c/gocloud/+/56810

Step 2: Submit a change to google.golang.org/protobuf that
adds the generated well-known types being migrated to that module.
In order to prevent the situation where a user links in
too old a version of the genproto module such that
duplicate registration occurs for the well-known types,
the registry is specially modified to provide an error
message that instructs users to upgrade the genproto module.
See https://golang.org/cl/234937

Step 3: Submit a change to google.golang.org/genproto that
switches all generated well-known types to be aliases to the
ones declared in google.golang.org/protobuf from the previous step.
This will cause the genproto module to incur an dependency
on an unreleased version of the protobuf module.
See googleapis/go-genproto#372

Step 4: Submit a change to google.golang.org/protobuf that
adds a weak module depdency on the genproto module at the
revision from the previous step.

Step 5: Release google.golang.org/protobuf@v1.24.0.

Step 6: Submit a change to google.golang.org/genproto that
updates the protobuf module dependency to v1.24.0.

…oogleapis#372)

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