Move `.pub` to `.dart_tool/pub`

[natebosch]

See dart-lang/site-www#444 (comment)

We are using .dart_tool/ directory for the build system because we didn't want to mix with .pub/.
This directory has another level of namespacing (we use .dart_tool/build today) which would let us add .dart_tool/pub and reduce the number of directories the user needs to ignore.

Moving would be good in the long term, but add some short term pain during the transition.

cc @kevmoo

[kevmoo]

👍– fewer directories is goodness.

[mit-mit]

re-titling to reflect that this wasn't just a discussion, the actual change was made