Generating malformed package under single-character directory

[gzzhanghao]

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When publishing npm packages under a single-character folder in root directory, fstream-npm generates malformed tarball package without the "package" directory in package.tgz.

Check out https://github.com/npm/fstream-npm/blob/master/fstream-npm.js#L355

When publishing a package under e:/t/foo, entry.path.substr(h.length + 1) evaluates to /foo and entry.path then becomes e:/foo, while it was expected to be e:/package/foo.