R1:18f049a282f4

Fix reading of the request path when running the PHP builtin webserver

Summary:
Ref T13575. Since PHP builtin webserver support was added, the pathway for parsing request parameters became more complex. We now rebuild "$_REQUEST" later, and this rebuild will destroy any mutations made to it here, so the assignment to "__path__" is lost.

Instead of "validating" the request path, make this method "read" the request path and store it explicitly, so it will survive any later request mutations.

Test Plan…