os/exec: fix edge cases in Windows PATH resolution

- Ignore empty entries in PATH, like PowerShell does.

- If we resolve a path using an explicit relative entry in PATH,
  treat it the same as we do for the implicit "." search path,
  by allowing a later (absolute) PATH entry that resolves to the
  same executable to return the absolute version of its path.

- If the requested path does not end with an extension matching
  PATHEXT, return ErrNotFound (indicating that we potentially searched
  for multiple alternatives and did not find one) instead of
  ErrNotExist (which would imply that we know the exact intended path
  but couldn't find it).

Fixes #61493.

Change-Id: I5b539d8616e3403825749d8eccf46725fa808a17
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-race,gotip-windows-amd64-race,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/528037
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

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