Avoid finding false-positive symlinks and printing warnings for them

[TimoWilken]

If users build e.g. O2Physics first with a branch called "x-y", and then later with a branch called "x", then aliBuild would print warnings about not being able to parse symlinks.

This is caused by an overly-broad glob when listing symlinks.

Replace this glob with a regex modelled after the regex that parses symlink targets, to avoid these false positives.

[TimoWilken]

@vkucera This should get rid of the extraneous warnings you saw!

[vkucera]

@vkucera This should get rid of the extraneous warnings you saw!

Thanks @TimoWilken !

[ktf]

Could you cover also the case the symlink returns NOENT?

[TimoWilken]

Thinking about this a bit more, it's probably better to handle all cases where we can't access the symlink dir by continuing as if it was empty. That fixes the coverage as well.