Add some checks on .install root and rootexec fields

[rjbou]

This PR adds some checks on the target optional field : it shouldn't begin with another field prefix. There a warning when it is the case (and failure on strict mode)
From #6938 (comment)

Todo :

• update Pp.warnwith an optional argument to not display warning only on verbose mode
• fix toplevel & stublibs paths
• queued on Add root and rootexec sections to .install files #6938

[kit-ty-kate]

I would argue that having this check would be at best unnecessary and at worst annoying for users generating their .install file themselves. I can honestly see value in giving users the possibility to simply do:

root: [
  ... # everything here
]

it would make protyping a lot easier for small tools and scripts (e.g. generating opam packages from other package managers)