Dependency handling

[awvwgk]

Take two on the dependency handling (see #251 for a recursive implementation).

• create a dependency tree data type with serde functionality
• allow translation from and to TOML of dependency tree
• implement iterative fetching of each dependency level
• use dependency tree in build_model
• update functionality for fpm-update subcommand
• build_model should use the dependency tree exclusively

Related #121
Fixes #265

Proposed command line interface for fpm-update

Name
    fpm-update(1) - manage project dependencies

Synopsis
    fpm update [--fetch-only] [--verbose] [--clean] [name(s)]

Description
    Manage and update project dependencies. If no dependency names are
    provided all the dependencies are updated automatically.

Options
  --clean       Ignore an existing package cache file
  --fetch-only  Only fetch dependencies, do not update existing projects
  --verbose     Show additional printout

Examples
 1. Initialize all project dependencies

    fpm update --fetch-only

 2. Update a list of dependencies

    fpm update --verbose dep1 dep2

Dependency management strategy

1. dependencies are handled iteratively
2. current project is added as first project to the dependency tree
3. all nodes in the dependency tree visited and their dependencies are added to the tree
4. repeat step 3 until all dependencies in the tree are visited
5. create a cache of the dependency tree (build/cache.toml)

[awvwgk]

I getting toward a working version here, one issue I noted is that git tags are not reliable and we have to treat them like branch names for now. One can actually force push and delete a tag with git, making it impossible to tell for fpm whether or not a dependency is fetched correctly without retrieving upstram. Therefore, I moved the TOML-Fortran pinning from tag to rev.

Right now the version is fully functional, while doing still a bit of redundant work for the fpm-build model, effectively walking the dependency tree twice, but the depth first approach of fpm-build will always find the already fetched dependencies from the dependency tree initialization doing no harm. I'll clean this up soon, but open the PR for feedback already.

Cleaned up and ready for review.

[LKedward]

Thanks @awvwgk, this is a significant improvement. Works as expected and the implementation looks robust. 👍

My only comment is that it would be good to include fpm update in the various general fpm help texts:

fpm/fpm/src/fpm_command_line.f90

Line 310 in f9e67cf

help_list_nodash=[character(len=80) :: &

fpm/fpm/src/fpm_command_line.f90

Line 326 in f9e67cf

help_list_dash = [character(len=80) :: &

fpm/fpm/src/fpm_command_line.f90

Line 401 in f9e67cf

help_fpm=[character(len=80) :: &

[awvwgk]

Seems like I always overlook parts of the command line documentation, I have to make a checklist for creating new commands to make sure I get it added to all the right places for the next time. Thanks for the reminder.

[awvwgk]

I would like to move at least one of new commands (fpm-install / fpm-update) forward to focus again on the fpm-dist command. Let me know if there is anything blocking this PR in particular.

[LKedward]

Let me know if there is anything blocking this PR in particular.

I'm happy for this PR to move forward; it would be good to get a second review if possible.

[LKedward]

Since this PR will close multiple issues and allow work to move forward elsewhere I think this can be merged if there are no further comments. @awvwgk are you happy to merge when you're ready?

[awvwgk]

Sounds good, I will rebase and merge this branch later today. If there are problems coming up later we can still handle those separately.