Copied E repository parts

[ciaranm]

We should allow ERepository to be adapted, probably via master_repository and requiring location=/var/empty. Something like:

# Use the RHS to create a new category named LHS with the same packages
# inside it as RHS
copied_categories = \
    i686-pc-linux-gnu-sys-devel : sys-devel ; \
    i686-pc-linux-gnu-sys-libs : sys-libs

# Define new category names
extra_categories = \
    i686-pc-linux-gnu-sys-kernel \
    i686-pc-linux-gnu-sys-apps

# Use the RHS to create a new package named LHS with the same IDs
# inside it as RHS. The category must already exist.
copied_packages = \
    i686-pc-linux-gnu-sys-apps/blah : sys-apps/blah

# Define new packages. The category must already exist.
extra_packages = \
    app-misc/foobar

# Define copied IDs. The best version of RHS is used to make LHS. The package
# must already exist.
copied_ids = \
    app-misc/foo-1.23 : >=app-misc/foo-1.0 \
    app-misc/bar-1.34 : =app-misc/foo-1.34