.pub -> .dart_tool/pub

[kwalrath]

Soon the place where pub hides its files will change (see dart-lang/sdk#32030). We need to update pages and scripts that refer to the old place. I found the following mentions of .pub:

• site-www/src/_guides/libraries/private-files.md:.pub/
• site-www/src/_tutorials/dart-vm/get-started.md:.pub
• site-www/src/tools/pub/cmd/pub-get.md:packages directories, or .pub directory (when present)

I should also fix, site-www/src/tools/pub/installing.md, which refers to .pub_cache (but should presumably be .pub-cache; I believe ~/.pub-cache is unaffected).

[bcko]

@kwalrath @chalin
The first two pages are already updated as shown below.

https://github.com/dart-lang/site-www/blob/master/src/_guides/libraries/private-files.md

Don’t commit the following files and directories created by pub:

.dart_tool/
.packages
build/
pubspec.lock  // Except for application packages

https://github.com/dart-lang/site-www/blob/master/src/_tutorials/dart-vm/get-started.md

Some of the files and directories in the hello_world app include the following:
.dart_tool
Support files used by pub and other Dart tools. You can ignore this.

I wasn't sure about the third page.
The third page mentions doesn't mention .pub but .pub-cache and packages.

[kwalrath]

Thanks for checking, @bcko! I'll take a look at the third page.

[kwalrath]

I just took another look at these pages, and the references to .pub* filenames all seem to be correct now.