One cannot have "external" top-level directory and more

Unfortunately if one have a directory named "external" it produces some kind of race (creating junction vs creating bazel internal directory with the same name) and often source forest creation **fails**:
```
ERROR: Source forest creation failed: Cannot create junction (name=C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\external, target=C:\users\chebotarev_v\perforce\chebotare
v_pdk\bin\bazel\external): ERROR: src/main/native/windows/file-jni.cc(86): nativeCreateJunction(C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\external, C:\users\chebotare
v_v\perforce\chebotarev_pdk\bin\bazel\external): ERROR: src/main/native/windows/file.cc(128): CreateJunction(\\?\C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\external):
Cannot create a file when that file already exists.
```

After second try it turns out that bazel erases source forest from bazel-bazel but **skips** directories which start from dot (see also #4060):
```
ERROR: Source forest creation failed: Cannot create junction (name=C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\.delivery, target=C:\users\chebotarev_v\perforce\chebotar
ev_pdk\bin\bazel\.delivery): ERROR: src/main/native/windows/file-jni.cc(86): nativeCreateJunction(C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\.delivery, C:\users\chebot
arev_v\perforce\chebotarev_pdk\bin\bazel\.delivery): ERROR: src/main/native/windows/file.cc(128): CreateJunction(\\?\C:\users\chebotarev_v\appdata\local\temp\_bazel_chebotarev_v\w0ulolcy\execroot\__main__\.delive
ry): Cannot create a file when that file already exists.
```

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### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I believe one can get fairly big project and create "external" and ".something" top level directories with some BUILD files and run `bazel build ...`. I will try `tensorflow` and post results later in discussion below.

### What operating system are you running Bazel on?

Windows 10 x64

### What's the output of `bazel info release`?

release 0.8.0 and release 0.9.0

### Have you found anything relevant by searching the web?

No.