Windows, macOS: package names and input-file-target names are case-ignoring

### Description of the problem / feature request:

Bazel does not enforce correct casing in package names, nor in target names (if they refer to input files).

Bazel only enforces correct casing in target names that are rules or output files. (Because these are defined as strings in the BUILD file.)

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

Repro on Windows -- in cmd.exe:
```
md c:\src\tmp\case\Foo\Bar

cd c:\src\tmp\case

type nul > WORKSPACE

echo Servus > Foo\Bar\Hello

echo genrule(name="x",srcs=["foo/bar/hello"],outs=["x.out"],cmd="(echo $< ; cat $< ;) > $@") > BUILD

bazel build //:x & type bazel-bin\x.out
(...)
INFO: Build completed successfully, 2 total actions
foo/bar/hello
Servus
```

Repro on macOS -- in Bash:
```
mkdir -p /tmp/case/Foo/Bar

cd /tmp/case

touch WORKSPACE

echo Servus > Foo/Bar/Hello

echo 'genrule(name="x",srcs=["foo/bar/hello"],outs=["x.out"],cmd="(echo $< ; cat $< ;) > $@")' > BUILD

bazel build //:x && cat bazel-bin/x.out
(...)
INFO: Build completed successfully, 2 total actions
foo/bar/hello
Servus
```

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

Windows 10
macOS 10.15.1

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

release 1.2.1

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

Same culprit as https://github.com/bazelbuild/bazel/issues/7627.