x/pkgsite: support build tags

[lazyhacker]

What is the URL of the page with the issue?

https://pkg.go.dev/lazyhacker.dev/getgo

What is your user agent?

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.114 Safari/537.36

Screenshot

What did you do?

Created a go project.

What did you expect to see?

In the source code section there should be a main_gui.go. In the directories section it should have internal > guimain

These are recognized by go mod as the go.mod shows the dpendencies that it needs.

What did you see instead?

It is missing the main_gui.go source file and internal library guimain.

[seankhliao]

related #37232

[jba]

The main_gui.go file has a gui build tag, so we don't show it.

While we support a few common build tags for GOOS/GOARCH, it isn't feasible to support arbitrary tags. The nature of our backend/frontend split requires that the backend know all the build tags that will be available to the frontend. #37232 explains the problem in more detail.