Improve new_lcg_version.py and add LCG 101a

To avoid special ad-hoc tricks for the special platforms I extended new_lcg_version.py with

• add handling of special platform flags
• fix use of platform aliases with automatic base (i.e. platform specifications like x86_64_v2-centos7-gcc11-opt= instead of x86_64-centos7-gcc11-opt=)
• add option to get the list of platforms from a file (i.e. platform specifications like @some_list_of_platforms.txt)

I also added the list of platforms needed for LCG_101a_LHCB_7.

To do:

• support for extension/override of lcginfo metadata (e.g for GitCondDB and custom DD4hep builds)
• create LCG_101a_LHCB_7 and LCG_101a (with LCG_101a an alias to LCG_101a_LHCB_7)

Supersedes !110 (closed)