x/pkgsite: README relative image links are broken if hosted on googlesource.com

[maruel]

Similar to issue #37194 but when the source code resolves to *.googlesource.com.

Repro:

• Host a project on googlesource.com
• Create a redirector, e.g. go.fuchsia.dev/shac-project/shac -> https://fuchsia.googlesource.com/shac-project/shac
• Include a relative image in the README.md, e.g. ![usage demonstration](images/demo.gif)

Example:

• https://fuchsia.googlesource.com/shac-project/shac/+/7fafae0f3d68b0a57a4532f7e2bd0fed7a2a1bf3/README.md
• https://pkg.go.dev/go.fuchsia.dev/shac-project/shac@v0.0.0-20230818210534-7fafae0f3d68

The latter has a broken image.

[findleyr]

Thanks for the report. I believe there's some hard-coded linking behavior for certain hosts, and we should add googlesource.com.