app.require_here: follow symlink'd main module

If a Lua application has, say, a main.lua and a lib.lua side-by-side,
and someone symlinks main.lua into (e.g.) ~/bin, we'd like
require_here() to look in the original directory, not ~/bin.

Symlink-following behavior can be disabled using the (new) second
argument to resolve_here.

CHANGELOG.md

@@ -7,6 +7,7 @@ deprecation policy.
 see [CONTRIBUTING.md](CONTRIBUTING.md#release-instructions-for-a-new-version) for release instructions
 
 ## 1.13.0 (unreleased)
+ - feat: `app.require_here` now follows symlink'd main modules to their directory
  - fix: `compat.warn` raised write guard warning in OpenResty
    [#414](https://github.com/lunarmodules/Penlight/pull/414)
  - feat: `utils.enum` now accepts hash tables, to enable better error handling

lua/pl/app.lua

@@ -29,12 +29,24 @@ end
 --
 -- Note: the path is prefixed, so it is searched first when requiring modules.
 -- @string base optional base directory (absolute, or relative path).
+-- @bool nofollow always use the invocation's directory, even if the main file is a symlink
 -- @treturn string the current script's path with a trailing slash
-function app.require_here (base)
-    local p = path.dirname(app.script_name())
+function app.require_here (base, nofollow)
+    local p = app.script_name()
     if not path.isabs(p) then
         p = path.join(path.currentdir(),p)
     end
+    if not nofollow then
+      local t = path.link_attrib(p)
+      if t.mode == 'link' then
+        t = t.target
+        if not path.isabs(t) then
+          t = path.join(path.dirname(p), t)
+        end
+        p = t
+      end
+    end
+    p = path.normpath(path.dirname(p))
     if p:sub(-1,-1) ~= path.sep then
         p = p..path.sep
     end