Safeguards for changes in lockfile/crate format (#436)

Safeguards for changes in lockfile/crate format (#436)

If the lockfile is not loadable we presume it is in an old format. In this case
we recreate it from scratch, which entails the loss of any stored pins.  If the
alire/crate.toml file is not loadable we advice the user to recreate the
workspace from scratch.

Note that both crate.toml and crate.lock use the index format to store
releases. Hence, any change in index format may impact these files. While the
lockfile is not that problematic, for the crate.toml file we have no easy
workaround since we cannot know to which crate the workspace belongs.

There are possible mitigations but as long as we don't have stable releases the
complexity/benefit ratio is unclear. If we eventually make the crate.toml file
only manually editable that could also impact how to deal with this issue, so
not doing anything about it yet.