cmd/go, go/build: parse directives in file headers
For #56986, go/build needs to report up to cmd/go
about //go:debug lines found in the source code.
Rather than make a special case for //go:debug,
this change gathers all top-level directives above the
package line and includes them in the result.
The go command's module index must match go/build,
so this CL contains the code to update the index as well.
A future CL will use the //go:debug lines to prepare the default
GODEBUG settings, as well as rejecting such lines in non-main
packages.