Handle expression/detected mismatch

While looking at #309 I noticed that the declared license on libssh2-sys is completely incorrect, the Rust code may be licensed as `Apache-2.0 OR MIT`, but the C code that it is building and wrapping is definitely not. Right now we assume that the declared license is correct, but that's obviously not true, particularly in the -sys wrapper case.

Start by tracing how cargo-about determines the declared license for libssh2-sys and review the context in #309. Determine how a mismatch between the Rust declaration and the wrapped C code should be reported or handled, then add coverage for that case once the expected behavior is decided.

Written by the indexing model from the issue text.