Associate docs from a metapackage to the -lib, -tests etc. sub-packages

[SuzanneSoy]

Given the following packages:

name: foo            tags: some-tag              has-docs: yes   implies: foo-lib, foo-doc, superfoo
name: foo-lib        tags: other-tag, foo        has-docs: yes   implies:
name: foo-doc        tags: third-tag, foo        has-docs: no    implies:
name: superfoo       tags: fourth-tag, foo       has-docs: no    implies:
name: foo-v2         tags: fifth-tag, foo        has-docs: no    implies:
name: scribble       tags: seventh-tag           has-docs: yes   implies:
name: scribble-bar   tags: sixth-tag, scribble   has-docs: no    implies:

this PR will show (in the main package list) the docs from foo and foo-doc in the package list entries for foo, foo-lib, foo-doc, superfoo. But it will not show the docs from scribble in the entry for scribble-bar.

The rule is:

• if metapkg implies subpkg,
• and subpkg has a tag named metapkg
• then subpkg will list the docs from metapkg (if any) and the docs from any other sibbling subpkg.

This is an initial fix for https://github.com/racket/racket-pkg-website/issues/58. A later version could add a proper 'metapackage or 'docs-of etc. field as suggested in some posts in that issue.

[SuzanneSoy]

This should remove all the "This package needs documentation" to-dos, as long as the authors add to the sub-packages a tag with the same name as the main metapackage.

[jeapostrophe]

This looks good to me @tonyg