An asset whose path is a prefix of another should cause a collision

[munificent]

Since barback doesn't have a concept of directories, it's possible to emit two assets like:

foo/bar // a file "bar" with no extension
foo/bar/baz.txt

That will fail horribly with something like pub build when it tries to output those two files. Instead of allowing this, barback should probably treat that like a collision.

[lrhn]

Added Area-Pkg label.

[DartBot]

This issue has been moved to dart-archive/barback#36.