scons: Build the source filter factories dict in SourceFilter.
scons: Build the source filter factories dict in SourceFilter.

This is a little cleaner since it avoids an additional global variable.

Change-Id: I19d9a0afd12fdfeeda0b524bd71943d155ed5d7d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48375
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

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