Windows,Bazel client: check embedded tools faster

The Bazel client on Windows is now 50% faster to
check the embedded tools than it was before.

Results:
- Linux: 20 ms -> 6 ms
- Windows: 294 ms -> 133 ms

Measurements were done with n=10 runs and a hot
server, using blaze::GetMillisecondsMonotonic().

Previously the client performed the same tasks
multiple times while trying to determine if a path
was a good extracted binary. (E.g. converted the
path to Windows format multiple times, checked if
it was a directory twice, opened the path twice.)

Now the client performes these tasks only once,
e.g. it converts path once and stats only once.

See https://github.com/bazelbuild/bazel/issues/5444

Closes #5445.

PiperOrigin-RevId: 201913758

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