x/pkgsite: support searching by <package>.<identifier>

[matttproud]

What is the URL of the page with the issue?

https://pkg.go.dev/search?q=sql.DB

What is your user agent?

Mozilla/5.0 (X11; CrOS x86_64 13597.66.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.109 Safari/537.36

Screenshot

What did you do?

I wanted to view the documentation for sql.DB, which is part of the standard distribution. I entered the search query of sql.DB into the interface, and it gave me no good results.

What did you expect to see?

Basically this but on the new web site: https://golang.org/pkg/database/sql/#DB

What did you see instead?

I don't know how the web application is implemented, but I expect either standard distribution packages are not indexed, the search query format "<package name>.<Identifier>" is not supported, or the result ranking is somehow deficient. I had been using this on old Godoc with great success since time immemorial.

[magical]

I'd go further and allow searching for just <identifier>, at least within the standard library. I occasionally use golang.org's search to find, e.g., a function that i can't remember the package for https://golang.org/search?q=WriteFile#Functions, or all the types that implement some method https://golang.org/search?q=Sum#Global.