Doc links should use "color: inherit" style.

That way, they look like the color for the type of syntax element they
are, not the color of a link.
Various changes:

- Refactor code.

- In the case where a symbol is provided by multiple modules, doc-uri
  returns a list of modules and URIs (instead of a list of
  modules). That way, a caller could provide a list of the choices for
  a user.

- Ignore Planet modules: They make xref-tag->path+anchor blow up
  because they're not something path-element->string can handle.

- Handle xref-binding->definition-tag returning #f.