Generate bindings for wasi:cli/imports, not wasi:cli/command

[alexcrichton]

Closes #640

[pchickey]

Just to be clear: this change works because, by eliminating LDFLAGS += -Wl,--skip-wit-component in the build of components against this libc, wasi-sdk's linker will insert a wasi:cli/run@0.2.0#run export pointing to the libc _start symbol.

[alexcrichton]

Right yeah, although the --skip-wit-component bit is only related to tests in this repository so dosen't affect anyone else. Everyone else though will use the _start symbol like before to get that all hooked up right.