Valid filenames have length limits!

ext4 filenames are at most 255 bytes.  vfat filenames are bit more
lax, but we're often saving them on ext4 through a FUSE daemon, so
limit them the same way.

Since package names are used as directory names, verify that they're
valid filenames.

Tests to verify behavior.

Bug: 18689171
Change-Id: If7df4c40d352954510b71de4ff05d78259c721ed