Allow CreateRelationship to work with FileAccess.Write (dotnet/corefx#35763)

Allow CreateRelationship to work with FileAccess.Write (dotnet/corefx#35763)

When adding a relationship InternalRelationshipCollection was creating a part
but not writing anything to it.  This caused an empty entry to be created in
the zip archive which later needed to be rewritten.  That's disallowed with
FileAccess.Write.  Instead, avoid creating the entry until it's content is
written.

When writing a relationship file InternalRelationshipCollection was attempting
to truncate any existing content.  This throws when the package is open  with
FileAccess.Write since the part stream is not seekable.  Don't do the seek
when package is open for FileAccess.Write since no pre-existing content is
possible.

In Package.PartExists don't attempt to get the part.  PartExists on .NETCore
only needs to query its local list of parts.  Since it isn't returning a
mutable part, it doesn't need to throw for packages opened with FileAccess.Write.

Avoid out of bounds exception when enumerating part keys. The fix that delays
creating the relationship part until its content exposed an issue in
DoOperationOnEachPart.  This was creating a copy of the part keys, but then
enumerating over the length of original list instead of the copy.  Since I
delayed the creating of a part until it was flushed this made the length change
while iterating the loop.  Since this code was using its copied list of keys
it should have never been using the original bounds.

Commit migrated from https://github.com/dotnet/corefx/commit/1822fec76a7bc02764ffa94c0e1f97b8ebfdbdea