`dub.selections.json` files are now looked up in parent directories too

In case the root package directory doesn't contain a `dub.selections.json`
file, dub now looks in parent directories too and potentially uses the
first (deepest) one it finds - if and only if that JSON file contains an
optional new `"inheritable": true` flag.

This allows using a 'central' `dub.selections.json` file for a repository
containing multiple dub projects, making it automatically apply to all
builds in that source tree if located in the repository root directory
(unless a local `dub.selections.json` overrides it).

Such an inherited selections file is never mutated when running dub for a
nested project, i.e., changes are always saved to a *local*
`dub.selections.json` file. E.g., when running `dub upgrade` for a nested
project.
