net-mgmt/prometheus2: Add missing frontend files

Pull in pre-compiled assets from the ports-assets/net-mgmt_prometheus2 GitHub
repository and use them when compiling Prometheus, enabling the new UI to
function properly.

While here, remove GO_PKGNAME which is not needed with USES=go:modules.

PR: 241925
Submitted by: David O'Rourke <dor.bsd@xm0.uk> (maintainer)
Approved by: tz (mentor, implicit)