Allow using boostdep with older boost versions

In an effort of migrating from bintray downloads (with caching) on CI I wanted to use boostdep with older boost versions (Use case: Install a subset of the libraries)

The problem I'm running into is that some modules didn't exist yet. So for example FileSystem includes `boost/functional/hash_fwd.hpp` which is mapped to `container_hash` via exceptions.txt. But that module doesn't exist so the subsequent `git submodule update` fails.

A potential (partial) solution is to check the modules in the exceptions file via `is_module` and skip the following mappings if the module doesn't exist.

Downside: Moved headers such as in https://github.com/boostorg/boostdep/commit/9d5890819358d25d4482120650bc38e1199d4ebc#diff-6935182ccded16172f0fa2c6a34a6bb4 or https://github.com/boostorg/boostdep/commit/b87f4a3bdc316c354121057508d45c9c161ca28d#diff-6935182ccded16172f0fa2c6a34a6bb4 won't be correct.

But at least it runs through which is a bit better and such known things could be manually specified by the user