x/pkgsite/cmd/pkgsite: remove most external dependencies

[matloob]

We'd like to get cmd/pkgsite to a point where it can be used to provide documentation from the go command. To do that we'd have to remove most of its external dependencies to a point where we can integrate it into part of the go command. Currently a lot of the dependencies are not necessary for the functionality of cmd/pkgsite but are needed for the frontend. We should be able to inject a lot of the dependencies and remove others so that the build of cmd/pkgsite has very few dependencies so that it could be vendored into the go repo.

[gopherbot]

Change https://go.dev/cl/514521 mentions this issue: internal/middleware: move stats to its own package

[gopherbot]

Change https://go.dev/cl/514519 mentions this issue: internal/vuln: remove the dependency on golang.org/x/exp

[gopherbot]

Change https://go.dev/cl/514518 mentions this issue: internal/frontend: add an interface for creating request caches

[gopherbot]

Change https://go.dev/cl/514675 mentions this issue: internal/middleware: delete Language and languageTag

[gopherbot]

Change https://go.dev/cl/514520 mentions this issue: internal/frontend: remove experiment middleware from newTestServer

[gopherbot]

Change https://go.dev/cl/514522 mentions this issue: all: switch from github.com/ghodss/yaml to gopkg.in/yaml.v3

[gopherbot]

Change https://go.dev/cl/514676 mentions this issue: internal/frontend: remove dependency on cloud error reporting client

[gopherbot]

Change https://go.dev/cl/515376 mentions this issue: internal/config: remove dependency on MonitoredResource proto

[gopherbot]

Change https://go.dev/cl/515375 mentions this issue: cmd/log: move stackdriver logging to its own package