fix: longest-prefix selection in path_of_file library mappings

[ciaran-matthew-dunne]

path_of_file selects the library mapping whose filesystem prefix is the longest match for the file, but when a longer match replaced a previous candidate it paired the new module path mp with the previous candidate's shorter prefix p:

| Some(_, p) ->
    if String.(length p < length fp) then mapping := Some(mp, p)  (* should be fp *)

The remainder of the file path is then computed against the wrong prefix, so with nested mappings the module path gets a duplicated directory component.

Reproduction: checking an installed Stdlib file nests two mappings ([] ↦ …/lib_root and Stdlib ↦ …/lib_root/Stdlib) automatically.

$ lambdapi check $OPAM_SWITCH_PREFIX/lib/lambdapi/lib_root/Stdlib/Set.lp
File ".../lib_root/Stdlib/Stdlib/Set.lp" (or .dk) not found.   # path_of_file = Stdlib.Stdlib.Set

Fix: store the winning mapping with its own prefix, Some(mp, fp). The command above then computes Stdlib.Set and succeeds.

[fblanqui]

Thanks!