Consequently:

  o Changed Plugin.get_context() to a private Plugin._get_context() accessor.

  o Updated anything which imports Context to do so from private _context module

  o Updated docs to exclude the now private Context

I found myself writing the following split rules for an element:

    public:
      bst:
        split-rules:
          runtime:
            (=): []

          devel:
            (>):
            - |
              %{bindir}/*

The aim was to put all of this element's binaries into the 'devel'
domain, and make sure nothing went into the 'runtime' domain by
replacing the built-in rules for that domain with an empty list.

That wasn't working though because BuildStream was using [] (empty list)
as a sentinel to mean "do nothing", which is fine for the prepend and
append operators (prepending an empty list to a list has no effect) but
is not really correct for the overwrite operator.

This commit fixes that issue and adds a test.

We need to canonicalize the path using the `realpath -m` or
`realpath --canonicalize-missing`, otherwise if the directory doesn't
exist we get an error:

    realpath: /tmp/foo/bar/: No such file or directory

The tests still work if this happens but the sources go into the
default location because the BST_SOURCE_CACHE variable goes unset.

This required adding two new APIs to make up for it on the Source

  o get_project_directory()

    Added here because elements should not be accessing external
    resources, Sources needed for local files and GPG keys and such

  o translate_url()

    Used by sources to mish-mash the project aliases and create
    real urls.