fix: allow overriding default import map

[NtTestAlert]

Allow overriding defaultsByName either by explicitly setting the 0x00.. address in the import statement, by setting it to from "DYNAMIC", or by modifying the defaultsByName map (now exported)

Also including a fix for broken register-contract.cdc

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[NtTestAlert]

if the address is explicitly set it skips FlowManager.getAddress (so setContractAddress behavior slightly changed - shouldn't be a problem tho)

so might want to change it to only expose defaultsByName if this is not desired