x/pkgsite: add copy option for pseudoversions

[julieqiu]

What is the URL of the page with the issue?

Visited https://beta.pkg.go.dev/golang.org/x/tools

Screenshot

What did you expect to see?

Ability to copy the full version string for golang.org/x/tools

What did you see instead?

Typed "y" on the page to change the URL from https://beta.pkg.go.dev/golang.org/x/tools to https://beta.pkg.go.dev/golang.org/x/tools@v0.0.0-20201026223136-e84cfc6dd5ca, then copied full version string from my URL bar.

It would be useful to have a copy option for the version string on the main page.

[gopherbot]

Change https://golang.org/cl/276194 mentions this issue: content/static: changes incorrect versions link to versions page link & latest package version link