Fix the legacy package not handling correctly the loading of platforms

[silvanocerza]

Right now the legacy package handle the loading of platforms, both those installed via the CLI or the IDE, and those installed manually in the sketchbook hardware folder.

As of now manually installed platforms are unversioned, meaning the platform.txt version property is not parsed. Unversioned platforms have their version string set to "".

This causes some issues to implement some new features, like the core list --all command to list all installed and installable cores, so we want to read the version when loading the manually installed cores too.

The platforms documentation specifies that name and version properties must be set so we can be sure that they'll be there and can be read.

With #1166 I tried to introduce the --all flag for core list command but it caused some issues in the legacy packaged, since the tests were expecting an unversioned library they broke.

The way they broke made me notice that certain tests on the legacy package are not respecting the platforms documentation, since they work even if the platform.local.txt and the boards.local.txt are not stored at the same directory level as their counterparts platform.txt and boards.txt.

We must fix the tests in the legacy package so that the local.txt files are where they must be as per documentation and update the hardware loading accordingly if necessary.