feat(buildGo): Add support for building "external" Go libraries

Adds a buildGo.external function that can build packages following the
default go-tool package layout. Dependencies work the same way as they
do for other buildGo-packages, but instead of being passed straight to
the compiler a fake GOPATH is assembled using a symlink forest.

External currently supports very few direct configuration options and
was primarily created to build the protobuf packages, but it is also
useful for including external dependencies in buildGo-native projects.

The previous complex build logic for the protobuf package has been
replaced with a call to `external`.

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