Make pub a pub package

[DartBot]

<img src="https://avatars.githubusercontent.com/u/654526?v=3" align="left" width="96" height="96"hspace="10"> Issue by financeCoding
Originally opened as dart-lang/sdk#9456

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In https://github.com/dart-gde/discovery_api_dart_client_generator we have a dart script that automatically versions and commits code to github. Later the script uploads to pub via Process.run() commands. What we have works but a solution where we could import some 'package:pub/...' to handle this for us would be great. I also find that pub has a lot of common and reusable code that others might want to depend on. Pub's design is nice since everything is a library. I have ripped code out of pub for other projects.

https://github.com/financeCoding/discovery_api_dart_client_generator/blob/d263122e29774bc69ab9b84da58837026a6ec88e/tool/update.dart

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2108507?v=3" align="left" width="48" height="48"hspace="10"> Comment by dgrove

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Set owner to @munificent.
Removed Type-Defect label.
Added Type-Enhancement, Area-Pub, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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What we have works but a solution where we could import some 'package:pub/...' to handle this for us would be great. I also find that pub has a lot of common and reusable code that others might want to depend on. Pub's design is nice since everything is a library.

So there's two separate goals here: 1. A programmatic API to interact with pub. I think #­2435 covers that.

1. Various useful functionality in pub being available as packages.

We try to do this in general. oauth2, http, scheduled_test, stacktrace, and pathos are all packages of functionality we needed for pub that are available outside of pub. If there's other stuff in pub you'd like to see pulled out, please do file bugs for it. We like code reuse.

In the meantime, I'll mark this as a dupe for the talking to pub itself side.

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Added Duplicate label.
Marked as being merged into #35.