stddoclet: For -linksource pages, add links between members, classes & packages

The source files generated by -linksource would be much more useful
if each page contained internal links for all program elements.

It seems this should be do-able for those program elements
whose Doc objects we have, and use something like @link for
those whose Doc objects we don't have.

Carried to an extreme, this would essentially duplicate
many links already in the normal HTML pages.
It could contain the following links:

- Declarations (return types, argument types, field types)
- Implementation (types, members)
- Doc comment tags would not be interpreted, but would have
  links around them: @see, {@link}, {@linkplain), {@inheritDoc}, @throws
- package statements should link to package.html
- unsure how to link to overview.html

I imagine it would only add links, not text, so it would not
add "Specified by" links (to members in interfaces),
"Overrides" links (to members in classes), "Implemented interfaces", etc.

We might want to add an option to suppress these links
if they increase the size of the entire document (including
normal javadoc HTML pages) by a substantial amount (say 30%).

###@###.### 2002-04-25