x/pkgsite: add support for plain text interface

[Zamony]

As far as I can tell pkg.go.dev doesn't support this feature either
https://github.com/golang/gddo/wiki/Plain-Text-Interface

What did you do?

curl -H 'Accept: text/plain' https://godoc.org/math

What did you expect to see?

Plain text representation of math package

What did you see instead?

<a href="https://pkg.go.dev/search?q=sql&amp;utm_source=godoc">Found</a>.

[seankhliao]

due to #43178

cc @julieqiu

[fzipp]

I noticed this too when gofind stopped working. It used godoc.org's plain text interface, and I updated it to scrape pkg.go.dev's search results HTML page for now - until pkg.go.dev gets an API (#36785).

[gopherbot]

Change https://golang.org/cl/288949 mentions this issue: gddo-server: do not redirect plain text requests