Fix derive_dialect! proc macro for use from external crates

[alexander-beedie]

Unfortunately I made a classic proc-macro blunder in #2174, forgetting that $CARGO_MANIFEST_DIR points to the crate being compiled, which is not the sqlparser crate. So all the tests worked (as the crate being compiled and the sqlparser crate are the same), but actual use from an external crate fails (without exotic workarounds) 😅

This PR fixes and industrialises the source lookup; now it does the following (documented/explained in the code too):

1. $CARGO_MANIFEST_DIR/src/dialect/mod.rs: works when macro invoked from within the sqlparser crate (e.g. tests).
2. <sqlparser_derive dir>/../src/dialect/mod.rs: works when sqlparser_derive lives in a workspace alongside the main crate (standard derive/ layout).
3. Sibling directories of compiled sqlparser_derive crate in the Cargo registry: works when an external crate uses derive_dialect! via registry dependency.

Confirmed this addresses the issue, and dialects can now be properly derived in/from external crates 👍
Also: minor tweak to use slightly more idiomatic map_err! inside derive_dialect.