The parseURL variable was introduced in CL 49930 in order to work
around the fact that the name "url" was shadowed by a parameter of
exported functions, and couldn't be renamed without sacrificing
documentation readability. Documentation readability takes higher
priority than internal implementation details.

Back then, I considered renaming the net/url import but saw that it
would be too disruptive of a change to the large net/http package.

Now I see a better way: it's possible to import net/url both as url
and as urlpkg (the package is still imported just once, but it becomes
available via two names). This way we eliminate the need for wasting
(a little) memory on the parseURL variable, improve code readability
slightly, and delete some lines of code and comments.

Updates #21077

Change-Id: I42cd9833afdcf4a5f5874fb7ee9c8c11eae557dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/202482Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>