lib.getExe: Do not make assumptions about the main program

Before this commit, getExe assumes that if `meta.mainProgram` is unset,
it has a main program that's named after the package name.

While this is probable, it leads to a bad error when the assumption does
not hold. If the user called `getExe` themselves, they might narrow down
the location of the assumption quite easily, but if that's not the case,
they'll have to go down the rabbit hole to figure out what went wrong.

For example, a NixOS module may use `lib.getExe` on a package-typed option
which is then used in the system configuration. This then typically leads
to a failure *after* deployment, which is bad, and it's quite likely that
the user will debug the package output contents before digging through the
NixOS module, which is bad.
Furthermore the `getExe` call is rather inconspicuous as it does not
contain something like "/bin/foo", which is bad.
Also modules can be hard to read for a newbie, which is bad.

All of this can be avoided by requiring `meta.mainProgram`.
Many packages already have the attribute, and I would expect 80% of
`getExe` usages to be covered by 20% of packages, because plenty of
packages aren't used with `getExe` anyway.

Finally we could make an effort to set `mainProgram` semi-automatically
using `nix-index`.

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