net/url: parseHost could share code with net/SplitHostPort

[jawnsy]

It seems like the code for parsing hosts in net/url - the parseHost method:
https://github.com/golang/go/blob/master/src/net/url/url.go#L540

could invoke the net/SplitHostPort method instead:
https://github.com/golang/go/blob/master/src/net/ipsock.go#L109

Is the separate parseHost done for performance optimization reasons, or was this just an oversight? If the former, maybe the code would benefit from a comment explaining this (and referencing the other method).

[bradfitz]

The net/url package doesn't depend on the net package. See comments in src/go/build/deps_test.go.

Doesn't seem worth refactoring into a third shared package to share code either.