Perform an initial checkout if library is missing

[jfrimmel]

As mentioned in #603, it is tempting to check in the fusesoc.conf-file into a version control system. This way all the necessary libraries are written down explicitly and changes are tracked as well. Note, that this is not the intended workflow, but it almost works with fusesoc version 2.4.2, except for one usecase:
in a clean checkout of a repository, there is the fusesoc.conf, but crucially not the fusesoc_library. In this case, almost all fusesoc-commands will fail with an error like this:

$ ls fusesoc_libraries fusesoc.conf
ls: cannot access 'fusesoc_libraries': No such file or directory
fusesoc.conf
$ cat fusesoc.conf
[library.vlog_tb_utils]
location = fusesoc_libraries/vlog_tb_utils
sync-uri = https://github.com/fusesoc/vlog_tb_utils
sync-type = git
auto-sync = true

$ fusesoc library update
WARNING: Failed to register library 'fusesoc_libraries/vlog_tb_utils is not a directory'

When looking into the code, the error can be traced to a FIXME, which exactly describes the initialization of missing libraries that this commit tries to add.

All it does is to move the check for a non-existing directory downwards after there is the correct provider. If the path doesn't exist, it tries to initialize the library using said provider, failing with the old error output as before this commit. This way, missing libraries get initialized gracefully and the usecase described above is supported.

The new testcase is simple: it creates an empty library directory, which gets updated. Since the core directory does not exist, an initial checkout should be performed.

Fixes #603.

[olofk]

A very welcome addition. I must admit I don't follow the logic in the code but that's mainly because it has been a long time since I last looked at it. Your description of the issues makes it clear that you have thought about this so I trust your judgement.

Thanks for your contributions. Picked and pushed!