The Go language specification states that the package import path is an opaque token.
The Go team has decided to interpret that as an opportunity to interpret these tokens as an expression language with the following capabilities: locate packages on the filesystem, locate packages from some specific version control systems, (proposed) control how packages are compiled.
My beef with this change is two-fold. First, why is there no specification for the package import path when there is plain, obvious, and necessary information encoded in this opaque token. Second, why is expediency a good justification for including compiler control flags in this undefined, unspecified opaque token.
If the package import path is going to be used like this, then it should be standardized in the language specification.
The answer to your first beef is that it would complicate the specification unnecessarily. You would suddenly have to provide a lot more details in the language spec, such as defining what capabilities a filesystem must provide, environment variables, OS and architecture details, package formats, compiler invocation syntax, and so on. That's all taken care of by the go tool right now, and doesn't burden other compiler implementations with all that.
Expediency is not used as a justification for this at all. You're wrong that any compiler control flags get involved here; 6g is going to be invoked with exactly the same syntax, and doesn't get told anything about "internal", just as it isn't told anything about build tags, or OS/arch-specific filename control (e.g. foo_linux.go).
The Go team has decided to interpret that as an opportunity to interpret these tokens as an expression language with the following capabilities: locate packages on the filesystem, locate packages from some specific version control systems, (proposed) control how packages are compiled.
My beef with this change is two-fold. First, why is there no specification for the package import path when there is plain, obvious, and necessary information encoded in this opaque token. Second, why is expediency a good justification for including compiler control flags in this undefined, unspecified opaque token.
If the package import path is going to be used like this, then it should be standardized in the language specification.