TODO:

- [ ] Decide on the proper name of the "element" configuration option
- [ ] Check if this works in various corner cases
- [ ] Add tests
- [ ] Add docs

In the previous commit, we added support to express cross-junction
dependencies inline as simple strings. Document it along with the
version in which the feature was added.

Now that 'bst fetch' is obsolete, change guidance to refer to the
replacement 'bst source fetch' instead.

Now that 'bst track' is obsolete, change guidance to refer to the
replacement 'bst source track' instead.

Since python 3.3, collections has been moved to collections.abc module.
For backwards compatibility, they continue to be visible in this module through Python 3.7.
Subsequently, they will be removed entirely.

See https://docs.python.org/3/library/collections.html

Fixes #424

  o Now the page titles are declared in plugins, allowing for
    a more descriptive ToC

  o Makefile and plugin.rsttemplate updated to not produce the title,
    to no longer use `:orphan:` for plugin pages, and to ignore any
    private modules in the plugin directories.

  o Interestingly, now the docs will fail to build if you add
    a new plugin and forget to add it to the documentation.

Move this logic into the junction element itself, instead
of special case erroring for this in the loader.

Plugins set their attributes in configure(), because the
constructor is not public API.