tvix/glue: builtins.placeholder input validation
#373
Opened by flokli at
Spun out of https://cl.tvl.fyi/c/depot/+/10200/comment/dfc84c25_a78bea70/.
builtins.placeholder has the same restrictions for output path names as Derivation has, so the code for this should be factored out and reused here, too.