package_repo: Fix update command when used with relative target.

Renaming of the temporary repo file failed when used with a relative
target path. The path was appended twice in such a case. Also when
used with a specified base directory, the relative temporary repo file
wasn't reachable from the changed working directory.

To fix both cases, create the BEntry pointing at the temporary repo
file before changing the directory and rename it using only the target
leaf name.

Fix behaviour of 'package_repo update':

* Instead of copying over the repository info from the source
repository, we need to read the repository info from a repository
info file corresponding to the target repository. Among other
problems, copying over the repository info from the source repo
put the wrong repository URL into the new repository, which in
turn caused the package resolution done by Haiku's build system to
fail (it tried to download packages from the wrong URL).
Additionally, I have updated the incorrect repository on the server, so
building Haiku-x86 should work again.

Actually rename the updated repository to the target name.

Fix keeping of package info during repository update.

* actually write the package info read from the repository, not the one
from the package, as the latter is missing the checksum

Use BPackageInfoContentHandler in package_repo 'update'.

* Ingo has pointed me at this class which already does the collecting
of package attributes into a PackageInfo - so there's no need to
do it manually.

Add command 'update' to package_repo.

* 'update' takes an existing repository and a package list file and then
creates a new repository from that, avoiding expensive
checksum-recomputations where possible

Fix behaviour of 'package_repo update':

* Instead of copying over the repository info from the source
repository, we need to read the repository info from a repository
info file corresponding to the target repository. Among other
problems, copying over the repository info from the source repo
put the wrong repository URL into the new repository, which in
turn caused the package resolution done by Haiku's build system to
fail (it tried to download packages from the wrong URL).
Additionally, I have updated the incorrect repository on the server, so
building Haiku-x86 should work again.

Actually rename the updated repository to the target name.

Fix keeping of package info during repository update.

* actually write the package info read from the repository, not the one
from the package, as the latter is missing the checksum

Use BPackageInfoContentHandler in package_repo 'update'.

* Ingo has pointed me at this class which already does the collecting
of package attributes into a PackageInfo - so there's no need to
do it manually.

Add command 'update' to package_repo.

* 'update' takes an existing repository and a package list file and then
creates a new repository from that, avoiding expensive
checksum-recomputations where possible