Directory dir_exists fails on exported projects on subpaths & paths with more than 2 slashes in res`

[Mantarri]

Godot version:
3.2.1

OS/device including version:
Arch Linux, up to date as of April 6th, 2020

Issue description:
Directory, and File classes fail to find directories, or files, using dir_exists, file_exists on exported projects.

Steps to reproduce:

Create Directory object, and attempt to check whether directory/file exists, note that it shows the file/directory exists when the project is run from the editor, but shows that the file/directory does not export when project is exported, and the resulting binary is run.

Note: dir.open does work with the same path, so it seems to be exclusive to how dir_exists work.

Minimal reproduction project:

Sorry for the OneDrive link
There are similar issues (including 26009), but was directed to create my own, so here it is.

[dankan1890]

It seems that in the export settings miss the inclusion of the file *.txt.

[Mantarri]

It seems that in the export settings miss the inclusion of the file *.txt.

My bad, I fixed that, but it still seems to have problems with finding directories at concatenated paths, everything else works as intended though.

[Mantarri]

After more testing, it appears to work if I remove res:// for checking for directories in res://, but works if I start with user:// to check for directories in user://.

[Mantarri]

After even more testing, it appears that dir_exists is only working for directories directly inside res://, and as a nice tidbit, it will find res://aFolderIsHere, but fails if you add / to the end like this res://aFolderIsHere/

[rburing]

Here are the bugs:

godot/core/io/file_access_pack.cpp

Lines 507 to 517 in 3c42d57

	bool DirAccessPack::file_exists(String p_file) {
	p_file = fix_path(p_file);
	return current->files.has(p_file);
	}
	bool DirAccessPack::dir_exists(String p_dir) {
	p_dir = fix_path(p_dir);
	return current->subdirs.has(p_dir);
	}

The only thing that is done when dir_exists is passed an absolute res://... path which points into a PCK file (i.e. in exported projects) is removing the res:// prefix and checking whether that path (including slashes, if any) is a direct subdirectory. Of course, this will be false for any path containing more slashes than those those two in res://. We need to go deeper...

[akien-mga]

See #56737 (comment) where @timvisee found that a similar bug is fixed in master, and proposed a potential fix or workaround.

CC @RandomShaper