Automated dependency tree update

[perazz]

Fixes #837 #799 (also targeting #838).

tl;dr fpm does never update a project's cached dependencies unless forced to do so via fpm update.
This is wrong as dependencies may change in the manifest and the user should not have to manually do it every time.

With the current PR:

1. The ability to update dependencies is propagated to fpm build:
   https://github.com/perazz/fpm/blob/063148259fc4745f60c8e3d253c803b3e09ace38/src/fpm.f90#L63-L70

2. When cache.toml is present, the old dependency tree was only loaded from it. New dependencies would be added from scratch (id==0), but cached dependencies (id>0) would be left as-is. Now, instead of just finding the ID of a cached dependency, fpm will check if it has changed, and request an update if so
   https://github.com/perazz/fpm/blob/063148259fc4745f60c8e3d253c803b3e09ace38/src/fpm/dependency.f90#L373-L389

This resolves changes in fpm.toml automatically and should be easily integrated with the local registry cache (#817 @minhqdao). @certik please let me know about the module contents problems (#838), I'm happy to help.

[henilp105]

Thanks @perazz , look good to me.