add mirror requirements list

there are some packages we want in the mirror (at all versions)
however we don't want projects to be able to specify them directly,
because they are always going to have versions specified by other
libraries.

The current mirror process processes things that look like
requirements files from global-requirements, so let us specify
these additional mirror requirements here.

Change-Id: I7436cf84f58cabe97e6fa8df2be43bc44061d8ba