os/exec: LookPath considers paths containing ":" to be absolute on windows

[rolandshoemaker]

os/exec.LookPath considers all paths containing : to be absolute, as one use of the colon is to indicate drive letters. Unfortunately it is also used for "alternate data streams" on NTFS filesystems, which allow attaching additional data to a file, accessed using a special suffix.

This can result in LookPath returning a file (with a ADS) in the current directory unexpectedly.

This is a PUBLIC track security issue per our security policy.

Thanks to Juho Forsén for reporting this issue.