Need tests for code that determines what files are published with a package

[munificent]

The code for getting the list of files to include in a package has a bunch of interesting corners related to symlinks, git, etc. We've seen a few issues there where it doesn't give us the files we expect, and we don't have good test coverage for it. We should. In particular:

1. Make sure packages directories are excluded.
2. Make sure symlinks are handled correctly.
3. Make sure .gitignored files are ignored.
4. Make sure .gitignored directories (and their contents) are ignored.
5. Make sure "." files are ignored in non-git packages.

[munificent]

1. Make sure "pubspec.lock" is always omitted.

[munificent]

Removed Type-Defect label.
Added Type-Enhancement label.

[munificent]

Removed this from the M3 milestone.
Added this to the M4 milestone.

[nex3]

We should test this as part of our "pub lish" integration tests, but right now we have no way of parsing a multipart/form-data request.

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Marked this as being blocked by #2488.

[sgjesse]

Marked this as being blocked by #8834.
Unmarked this as being blocked by #2488.

[munificent]

Removed this from the M4 milestone.
Added this to the M5 milestone.

[nex3]

Set owner to @nex3.
Added Started label.

[nex3]

Fixed by r22807.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#377.