fix(stage): strip path separators from staged tarball filename

[ubeddulla]

npm stage download builds the output filename from the staged tarball's own package.json name and version, but those come straight out of the untrusted tarball (parsed raw, never validated), and the name only replaces the first @// while the version is dropped in verbatim. A staged package whose version is something like ../../../../tmp/pwned makes resolve(process.cwd(), filename) walk out of the working directory, so writeFile drops the attacker-controlled bytes at an arbitrary path. Strip every separator from the name and version before building the filename, the same way libnpmpack does, so it stays a single component inside cwd. Added a regression test that downloads a tarball with a crafted version and checks nothing lands outside the cwd.