Support importing an implementation contract

[ericglau]

forceImport only supports importing a proxy or beacon. Enhance this to support importing an implementation contract.

Example use case:

1. User previously deployed implementation contract manually
2. Import implementation contract so it will be in the manifest
3. Run defender.proposeUpgrade to propose an upgrade -- if the contract was imported above, it would not need be redeployed

Consider a new function forceImportImplementation separate from forceImport, to avoid users accidentally importing a proxy as an implementation. Use the same forceImport function for simplicity. Contract will only be imported as an implementation if it is not detected as a proxy or beacon.

[ericglau]

Will be handled as part of #580