Fix git dep, and fix name parsing from filename in cli (#384)

* drop `branch = "main"` from wasmtime git dep

this is implied since main is the default branch, and means that in other places
that depend on the default branch, the wasmtime brought in by these
crates is not the same dependency

additionally, drop the dep on wasmtime-wasi altogether, it is not used.

* wit-bindgen-cli: parse out name from before first `.` in filename

to handle `*.wit.md` files correctly.

Author: pchickey