Docs: get package names from purs graph

[f-f]

Fix #1065 as described in here

This patch started with just "let's thread the graph through the docs-search machinery", but getting that done snowballed into a few more things.
Brief of changes:

• we used to get package names from folders on disk, which didn't quite work. We now compute the purs graph instead, and pass it in
• that entailed splitting out the graph types into their own separate module so we could avoid circular imports
• however, the PackageName used in those graph types (the one from the Registry) was not the same PackageName used in the docs-search codebase (the one from Bower), so this second one is no more
• however that Bower stuff was getting in from somewhere; we were in fact parsing bower.json files to figure out package dependencies. That's just wrong, so that's no more, and we do instead parse purs.json files, which are accurate
• in passing I noticed that we were parsing the module names from the local modules. That probably worked ok in the old architecture, but with the new monorepo architecture it's just wrong. So the code for parsing the files is no more, and we instead just pass in the package names of the workspace packages and read the modules from the graph

[fsoikin]

Looks sparkly!