scons: Get rid of special handling of the _m5 package.
scons: Get rid of special handling of the _m5 package.

This package is handled specially by the DictImporter used during the
build, and an assert shows that that code is never actually used. That
makes sense, since _m5 won't be added to gem5 using the PySource
mechanism.

Change-Id: I36a39f1ebb94a7620c8ba296e0fe856bd33285f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48362
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5