wasm_bindgen: allow target name with directory

[matt-duch]

Looking at the current logic for setting --out-dir and --out-name, it seems that the intention is to infer those from the target name. However, if a prefix is specified for the name (pkg/custom), out-dir is set as expected (pkg), but that directory prefix is also set in out-name (pkg/custom). causing the build command to fail when it appends the two and ends up with pkg/pkg/custom).

This approach sets out-name to be the last path segment only.

[matt-duch]

I ended up needing to add the snippets directory, similar to #1343 (except that PR changes paths to include the name twice, not sure why). Adding the snippets is actually related to this change, so I could fold that in if that is of interest as well.

[illicitonion]

This generally LGTM, but two questions:

1. Can you do a string replacement of / -> _ rather than taking the basename, as it's more documenting and less likely to lead to conflicts?
2. Can you add a test (i.e. a target which didn't build without this change, and does with)?

Thanks!