internal/impl: rename encode_field.go->codec_field.go

The code organization is simpler if we keep the functions encoding and
decoding a particular type (e.g., maps) together rather than split
across files.

This rename is happening in a separate CL from cl/185241 to preserve
rename history. (Git gets confused when you rename a->b and b->c in the
same commit.)

Change-Id: Idfbb3ff8cf0db149c68d650f89ff3fb8ac833322
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/184942
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/encode_field.go b/internal/impl/codec_field.go
similarity index 100%
rename from internal/impl/encode_field.go
rename to internal/impl/codec_field.go