proposal: os/exec: use LookPathAbs by default

[rsc]

exec.Command("ls") calls LookPath to find an "ls" executable to run.
If users are on Windows, or if they are on Unix with "." in their $PATH ahead of /bin, it will run ./ls when that exists.
That's almost never what you really want - see #38736 and related issues.

If we adopt #42420, perhaps we should also make exec.Command default to using LookPathAbs instead of LookPath (unless the command is explicitly "./ls" (.\ls on Windows)).

[rsc]

We now have three active proposals related to dot in path lookup:

• proposal: os/exec: make LookPath not look in dot implicitly on Windows #38736, which removes the implicit dot lookup from LookPath on Windows
• proposal: os/exec: add LookPathAbs that refuses to return relative paths #42420, to add LookPathAbs that doesn't return relative results.
• proposal: os/exec: use LookPathAbs by default  #42950, to make exec.Command use LookPathAbs by default (assuming it is added)

Please try to keep comments on these limited to the specific issue they track. Thanks.

[rsc]

Retracting in favor of #43724.

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group