protoc-gen-go: remove generation of magic import comment

[dsnet]

This is a reversion of #267.

The Go tooling ecosystem in moving towards modules, where this import comment is being deprecated in favor of a go.mod file. The magic // import "path/to/package" comment may be used to initialize a go.mod file, but the use of go.mod supersedes the magic comment afterwards.

We may remove this now, or wait until module support is even more solidified.

[tandr]

Or at least make it configurable somehow

[dsymonds]

This move seems premature. While the package import comments might be deprecated, they will still be effective (and not harmful!) for likely quite some time. I'd wait at least a year on this.

[neild]

Are they particularly important to generate? We only started adding them relatively recently, and anyone who does want an import comment can include a handwritten stub .go file with just a package statement and the comment.

[dsymonds]

They're useful in the ways that import comments in non-generated packages are useful: they ensure a package is only imported in a specific way (and this is doubly important for proto packages to avoid duplicate registrations!).

I could write a stub .go file, but I've already written the package name in an option go_package directive in the .proto file.

Note that they existed for a long time internal to Google. I can't remember why it wasn't generated externally for so long. But my point still stands: they're useful (that's why you added them earlier this year!), and will remain useful for quite some time. Modules aren't going to be widely adopted in the short term; it'll take some time.

[neild]

I don't think the import comments ever existed internal to Google, did they? We build everything with Blaze, so option go_package never sets the import path--everything is explicitly set on the command-line and there isn't really a concept of importing something with the wrong path.