Switch build system from optional package to repositories

* Build libsolv and the dependency solver part of the package kit for
the build platform.
* Add build tool get_package_dependencies. Given a list of package files
and a list of repository files it determines the additional packages
that need to be retrieved from the repositories and prints their URLs.
* Add rules to work with external repositories in the build system
(build/jam/RepositoryRules):
- PackageRepository declares an external repository with all its
packages. The URL of the repository file isn't specified. It is
computed from a given base URL and the SHA256 hash of the list of
package files.
- GeneratedRepositoryPackageList generates a file containing the file
names of all packages in a repository.
- IsPackageAvailable returns whether a package is available in any
repository.
- PackageURL returns the URL for a package.
* Declare the HaikuPorts repository for x86_gcc2
(build/jam/repositories/HaikuPorts/x86_gcc2).
* Add rule AddHaikuImagePackages to add a package to the image and rule
IsHaikuImagePackageAdded to determine whether a package has been
added.
* OptionalPackages: Remove all entries that just downloaded and
installed an external package. AddHaikuImagePackages can be used
instead and is used in the remaining entries. Also move the remaining
optional package dependency declarations from
OptionalPackageDependencies here.
* ExtractBuildFeatureArchives: Instead of the URL parameter a package
name must be specified now. This allows to simplify BuildFeatures
significantly, since there's no dealing with URLs anymore. "if" out
the entries that aren't supported yet.
* build_haiku_image: For the packages installed in system and common
resolve their dependencies and download and install them as well.

Switch build system from optional package to repositories

* Build libsolv and the dependency solver part of the package kit for
the build platform.
* Add build tool get_package_dependencies. Given a list of package files
and a list of repository files it determines the additional packages
that need to be retrieved from the repositories and prints their URLs.
* Add rules to work with external repositories in the build system
(build/jam/RepositoryRules):
- PackageRepository declares an external repository with all its
packages. The URL of the repository file isn't specified. It is
computed from a given base URL and the SHA256 hash of the list of
package files.
- GeneratedRepositoryPackageList generates a file containing the file
names of all packages in a repository.
- IsPackageAvailable returns whether a package is available in any
repository.
- PackageURL returns the URL for a package.
* Declare the HaikuPorts repository for x86_gcc2
(build/jam/repositories/HaikuPorts/x86_gcc2).
* Add rule AddHaikuImagePackages to add a package to the image and rule
IsHaikuImagePackageAdded to determine whether a package has been
added.
* OptionalPackages: Remove all entries that just downloaded and
installed an external package. AddHaikuImagePackages can be used
instead and is used in the remaining entries. Also move the remaining
optional package dependency declarations from
OptionalPackageDependencies here.
* ExtractBuildFeatureArchives: Instead of the URL parameter a package
name must be specified now. This allows to simplify BuildFeatures
significantly, since there's no dealing with URLs anymore. "if" out
the entries that aren't supported yet.
* build_haiku_image: For the packages installed in system and common
resolve their dependencies and download and install them as well.