SynopsysTool and MentorTool belong in their respective cad-plugins repos

[jwright6323]

It probably makes sense to include the CadenceTool and SynopsysTool (and future InsertVendorTool) classes with their subclasses.

hammer/src/hammer-vlsi/hammer_vlsi/hammer_vlsi_impl.py

Line 370 in b8ef3a9

class CadenceTool(HasSDCSupport, HammerTool):

[edwardcwang]

Related to #198

[harrisonliew]

CadenceTool moved by #576 but SynopsysTool & MentorTool remain

[harrisonliew]

There needs to be a better mechanism for the respective plugin repos to import their common code. Currently, for Cadence tools, we have to do this:

sys.path.append(os.path.join(os.path.dirname(os.path.realpath(__file__)),"../../common"))
from tool import CadenceTool

[harrisonliew]

Solved in #713 and packaging infrastructure.