bugfixes to yarn pack, retry of #1464

[tkloht]

This is an updated version of #1464 which had to be reverted because of ci errors.
The errors were caused by changes in the yarn code after the PR was created (util/fs.copy() has an additional parameter).
This is fixed in this PR.

@bestander you asked me to ping you when this is done.

below is the original message of #1464:

Summary
Contains two fixes for yarn pack if "files" is used in package.json:

• some files (package.json, readme, license, changelog) should always be included, even if not listed in the files field. This was not the case, fixed by 13e7b42
• all other files should be excluded, but the exclude filter did not match files starting with a dot (this is the default behaviour of minimatch it seems). fixed by 8851acb

Please note that this only partly fixes #755 .
As far as I can see what happens in #755 is the following:

• some files which should be included we not included, in my example it was package.json
• some files which should be excluded were included, in my example .gitignore and others.

While working on this I was wondering why a missing file would create a corrupt archive, but it seemed to be fixed at first. Then I added some more testcases and noticed that it is still possible to create corrupt archives, but this is not limited to the usage of "files" - see #1442.
Basically I think some directory was falsely included, and that invoked the bug described in #1442 .
But a missing or included file does not directly result in a corrupt archive.
Hope this makes sense, I find it difficult to explain right now.

Test plan
I have added some tests for this, just run yarn test.
If you want to see the failing tests checkout 6612ec6.

[bestander]

Great job!

[tkloht]

thanks!

[garthk]

Just spotted this or a close relative in 0.18.1; will revisit when 0.19 drops.