Handle custom manifest name (#3000)

src/Types.jl

@@ -178,17 +178,24 @@ function projectfile_path(env_path::String; strict=false)
 end
 
 function manifestfile_path(env_path::String; strict=false)
-    for name in Base.manifest_names
+    man_names = @static Base.manifest_names isa Tuple ? Base.manifest_names : Base.manifest_names()
+    for name in man_names
         maybe_file = joinpath(env_path, name)
         isfile(maybe_file) && return maybe_file
     end
     if strict
         return nothing
     else
-        project = basename(projectfile_path(env_path)::String)
-        idx = findfirst(x -> x == project, Base.project_names)
-        @assert idx !== nothing
-        return joinpath(env_path, Base.manifest_names[idx])
+        n_names = length(man_names)
+        if n_names == 1
+            return joinpath(env_path, only(man_name))
+        else
+            project = basename(projectfile_path(env_path)::String)
+            idx = findfirst(x -> x == project, Base.project_names)
+            @assert idx !== nothing
+            idx = idx + (n_names - length(Base.project_names)) # ignore custom name if present
+            return joinpath(env_path, man_names[idx])
+        end
     end
 end