Highlight $(…)* and $foo in Vim.
Fix :syn-include usage of Vim filetype.

Here’s what the Vim manual says in *:syn-include*:

    :sy[ntax] include [@{grouplist-name}] {file-name}

	All syntax items declared in the included file will have the
	"contained" flag added.  In addition, if a group list is
	specified, all top-level syntax items in the included file will
	be added to that list.

We had two rules for `rustModPath`, one `contained` and the other not.
The effect was that the second (now renamed to `rustModPathInUse`) was
being included in the group list, and thus that all identifiers were
being highlighted as `Include`, which is definitely not what we wanted.
auto merge of #15450 : chris-morgan/rust/vim-misc-2014-07-04, r=kballard