build: vcbuild fix "The input line is too long."

vcbuild.bat is calling vcvars.bat, which doesn't detect if the environment
has already been set. This causes repeated entries to be added to the PATH,
which after a few invocations will lead to an error:
The input line is too long.
path: resolve normalize drive letter to lower case

make path.resolve work the same as path.normalize
deps: update uv to v1.0.0-rc2
core: replace use of uv_fs_readdir with uv_fs_scandir
core: fix usage of uv_cwd

It was modified in libuv to be consistent with uv_exepath and not
include the trailing NULL byte in the returned size.