Fix problems in path normalisation algorithm (luarocks#1541)

Current implementation of path normalisation contains various flaws:

- Trailing `.` and `..` at the end of path don't get normalised,
- Path `/` turns into an empty string after normalisation.

This patch changes implementation of normalisation to fix these.

(cherry picked from commit b8a2710)

spec/dir_spec.lua

@@ -52,12 +52,20 @@ describe("luarocks.dir #unit", function()
       it("strips unneeded /../ and /./", function()
          assert.are.same("/some/dir/file.txt", dir.normalize("/../../../some/./foo/bar/.././../dir/bla/../file.txt"))
          assert.are.same("/some/dir/file.txt", dir.normalize("/../../../some/./foo/bar/.././../dir/bla/../file.txt"))
+         assert.are.same("/some/dir", dir.normalize("/../../../some/./foo/bar/.././../dir/./some/subdir/../.."))
+         assert.are.same("/some/dir", dir.normalize("/../../../some/./foo/bar/.././../dir/./."))
       end)
       it("respects relative paths", function()
+         assert.are.same(".", dir.normalize("."))
          assert.are.same("boo", dir.normalize("./boo"))
          assert.are.same("/boo", dir.normalize("/./boo"))
          assert.are.same("../../../../boo", dir.normalize("../../../hello/world/../../../boo"))
       end)
+      it("respects root directory", function()
+         assert.are.same("/", dir.normalize("/"))
+         assert.are.same("/", dir.normalize("/////"))
+         assert.are.same("/", dir.normalize("/a/b/.././../c/./../../"))
+      end)
    end)
 
 end)

src/luarocks/core/dir.lua

@@ -63,6 +63,7 @@ function dir.normalize(name)
    if pathname:match("^.:") then
       drive, pathname = pathname:match("^(.:)(.*)$")
    end
+   pathname = pathname .. "/"
    for piece in pathname:gmatch("(.-)/") do
       if piece == ".." then
          local prev = pieces[#pieces]
@@ -75,11 +76,13 @@ function dir.normalize(name)
          table.insert(pieces, piece)
       end
    end
-   local basename = pathname:match("[^/]+$")
-   if basename then
-      table.insert(pieces, basename)
+   if #pieces == 0 then
+      pathname = drive .. "."
+   elseif #pieces == 1 and pieces[1] == "" then
+      pathname = drive .. "/"
+   else
+      pathname = drive .. table.concat(pieces, "/")
    end
-   pathname = drive .. table.concat(pieces, "/")
    if protocol ~= "file" then pathname = protocol .."://"..pathname end
    return pathname
 end