fix integration with echidna and medusa so that caching works

[0xalpharush]

We switched to only compiling foundry's source directory, excluding tests and scripts, in #488. This has interacted poorly with Echidna and Medusa. We should aim to have a way for Slither and fuzzers have the best possible defaults UX wise and most quickly recompile targets. Foundry now has caching for --build-info (foundry-rs/foundry#8164) and allows compiling individual directories/paths natively so we can clean up how we handle this (foundry-rs/foundry#8149).

We may need to have a way for Slither to not request the build info (including bytecode) and instead only get the AST (foundry-rs/foundry#7212, crytic/slither#2421).

xref foundry-rs/foundry#8071 (comment)

cc @aviggiano