Rewrite `importUntypedContract`

Description

Problem: the current implementation uses prepareContract that for me seems like a thing specific to morley executable (since it in theory allows reading a contract from stdin - we shouldn't use anything like that).

Another problem is that importUntypedContract effectively typechecks the contract and then throws out the typed contract. With the currently developed nettest where, as a baseline, we work with untyped representation and then provide typed layer above that, this may cause the same contract to be typechecked twice.

Solution: rewrite importUntypedContract similarly to the existing importContract for typed contracts.

Related issue(s)

Resolves #

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