VirtualFileSystem ls()/ls_all() ignores mounted ram disks

[Clockwork-Muse]

Description

When mounting a ram disk, calling ls() or ls_all() ignores ramdisk subdirectories

Steps to Reproduce

from panda3d.core import VirtualFileMountRamdisk, VirtualFileSystem

vfs = VirtualFileSystem.getGlobalPtr()
vfs.make_directory("somedir")

vfs.mount(VirtualFileMountRamdisk(), "somedir/subdir", 0)
# or:
# vfs.mount(VirtualFileMountRamdisk(), "somedir", 0)
# vfs.make_directory("somedir/subdir")

with open("somedir/subdir/somefile.txt", mode="wt", encoding="utf-8") as somefile:
    somefile.write("hello?")

print("root ls_all")
# prints nothing
vfs.ls_all("somedir")

print("subdir ls_all")
# if the mount point was `somedir/subdir`, this prints nothing
# if the mount point was `somedir`, then this prints `somefile.txt`
vfs.ls_all("somedir/subdir")

print(f"file exists: {vfs.exists('somedir/subdir/somefile.txt')}")

Environment

• Operating system: Ubuntu 20.04
• System architecture: x86_64
• Panda3D version: 1.10.11
• Installation method: pip wheel
• Python version (if using Python): 3.8.10

[rdb]

One problem is that has_file() returns false for the root of the ramdisk, which causes a bunch of problems. The other problem is a bug in VirtualFileSystem::scan_mount_points that caused mount points not to show up in the directory listing. Fixing for 1.10.15.