build: lua 5.1/5.2: use generic version names

TL;DR: use --lua=XXX for pkg-config name XXX, e.g. --lua=lua-5.1 .
       For unversioned 'lua.pc', use the name luadef51/luadef52 .
       Autodetection remains the same (5.2 names, luajit, 5.1 names).
       The old names are still supported, but not auto-detected.

Before this patch, if one wanted to choose a specific lua version when
more than one is installed, then the names were a mess, e.g. 51obsd is
also the name detected on Arch linux, and other (distro) names are
also not unique to a specific distro/platform.

So to ask mpv to choose the package name (specifically, the pkg-config
file name), one needs to look at the mpv sources and find the
(arbitrary) distro name which has the same lua version naming as they
do on their own system, e.g. --lua=51obsd on Arch. This is a pain.

Now we add generic names:
- luadef51/luadef52 - generic pkg-config lua.pc (version is inside).
- lua* - exactly the pkg-config name, e.g. --lua=lua-51 for lua-51.pc
  (the names are curated, e.g. --lua=foo won't detect foo.pc).
- The legacy names (e.g. 51deb) are still supported, but undocumented,
  and the new generic names take precedence during auto-detection.

The fact that the generic names all start with "lua" has an additional
benefit that it shows right after "lua" at the output of mpv -v,
while the old names start with numbers, so they're first at the list,
making it hard to understand that e.g. "51obsd" is the lua version.

None of these names are actually used at the mpv code. The C code
checks the version using the lua headers (LUA_VERSION_NUM).

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