[CMAKE] Rename the top-level and src/ cmake projects

Summary:
The same project name (BitcoinABC) is actually used at both level.
This would cause the src/ project properties to override the top-level
project properties.
The new names are chosen to be consistent with the other cmake project
names.

Test Plan:

cmake -GNinja ..
ninja

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4027