Update package name in debian/control to bitcoinabc

Summary:
Most of our package naming is bitcoin-abc, but the currently deployed packages in the Ubuntu PPA
are named as bitcoinabc. This patch makes us consistent with what is currently deployed until a strategy
for updating the source package name with minimal disruption to users is more clear.

Test Plan: Among other patches, this change was used to manually deploy a test package to https://launchpad.net/~jasonbcox/+archive/ubuntu/bitcoin-abc/+packages

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5461