SMTChecker: Fix analysis for selected contracts

[blishko]

Previously, when a contract was selected for analysis, the analysis was incorrect. There were two issues. First, the contracts in the same file were considered as entry points even though they were not selected for analysis. Second, the contracts in a different file were mostly ignored, resulting in unsoundness when an external call was made to such a contract in trusted mode.

The solution to the above problems is to always create representation of all contracts (in case they are called from the selected contract), but create verification targets only for the selected contracts.

Fixes #15836. Fixes #14275.