x/pkgsite: add statistics about module usage

[dsnet]

Thanks for the great work on go.dev website!

A given package has a "Imports" and "Imported By" tab, which allows you to get a sense of the dependency graph for a single package.

I propose adding a similar feature for modules, such that it would gain a "Requires" and a "Required By" tab that similarly presents the following information:

• For the "Requires" tab, it shows the set of modules required by the latest version of this module.
  • For example, the latest version of github.com/golang/protobuf depends on 3 other modules. I'd expect these to be readily displayed without needing to go to the go.mod file.
• For the "Required By" tab, it shows the set of modules at their latest version that requires this module.
  • For example, the github.com/golang/protobuf module is required by 8577 modules. I'd like to see this number and get a brief sample of some of these modules.
  • It'd be additionally be useful if the "Required By" tab could show the dependencies on specific versions of this module. For example, I'd like to easily see statistics similar to the following:
    • 1435x (16.7%) at v1.4
    • 6199x (72.3%) at v1.3
    • 721x (8.4%) at v1.2
    • 77x (0.9%) at v1.1
    • 45x (0.5%) at v1.0
    • 100x (1.2%) at v0.0

At the present moment, I obtain such information through a series of expensive queries on the module proxy.

[cagedmantis]

/cc @julieqiu

[julieqiu]

Thanks for the suggestion, @dsnet! I think this is useful information to have. One of the motivations for creating the /mod namespace was to add these tabs later on.

We are currently working on a redesign of the site, and can work on figuring where this information might belong.

/cc @spf13 @fflewddur @Joanne881107 for product/UX input