correct imports

Integrate the "dub select" functionality in "dub upgrade".

Make some cosmetic changes to dub version selections. ...

- Adjust code style
 - Let "dub select" output a simple package->version map instead of package->{version, dependencies}
 - Rename dub.select.json to dub.selections.json.

Preparing dub select command. dub select will select the currently used versions and write them to a file, so that dub can later use this information to rebuild this exact state. This will also be used to override dependencies with either branch versions or not matching versions (to resolve conflicts). (All of this if to be implemented). ...

Other changes:
- Added Version.UNKNOWN, which could be later used for undetermined package versions instead of ~master.
- Added Dependency to/fromJson.
- Added Dependency alias "*" for ">=0.0.0".

Not catching Throwable when metadata is fetched. ...

Documentation updated.

Add --force-remove switch and reimplement PackageManager.remove. ...

remove() now first checks for untracked files and leaves the package untouched if it finds some and no force_remove is supplied. This means that after the user gets an error concerning untracked files, it's possible to rerun dub with --force-remove to remove the package cleanly.

Avoid using the deprecated canFind function.

The externally defined sub-packages don't have the name declared in the base package. This eliminates the double definition of a sub-packages name.

Fix Project.getDefaultConfiguration to take the whole dependency tree into account.

Fix the configuration resolution algorithm. ...

The initial graph of the old algorithm contained edges to configurations that
couldn't themselves resolve all dependencies. These are now skipped, so that
only valid sub graphs exist. This avoids bogus "Could not resolve configuration
for package X" errors.

Keeping track of the saved json file in the journal and some smaller changes.

Make "dub.json" the primary choice for a package description and handle "package.json" as a fallback.

Subpackages, which are defined external to the main package.json can be fetched from the registry. Eventually, the dependencies of all referenced packages will be fulfilled, but this is done by a hack where the metadata of the external package is inferred as having no additional dependencies. When the package is then fetched the state is re-evaluated and possible new dependencies will be resolved.

Fix compiler errors on DMD 2.063.2.

Implement individual building of dependencies.

Warn about missing "mainSourceFile" only for executable configurations.

Avoid casting RegistryPackageSupplier[] to PackageSupplier[]. Fixes #186.

Make "dub test" work for library configurations without a "mainSourceFile".

Use an existing "unittest" configuration for "dub test" by default.

Fix "mainSourceFile" to be emitted for Project.addBuildSettings with the "shallow" parameter set. ...

Fixes "dub test" for library targets.

Implement the "test" command.

Output a warning if no main source file is defined.

Revamp the command line interface with cleanly separated command implementations.

Avoid fetching the same package multiple times when sub packages are used.

Fix naming convention of PlacementLocation.

Only install pre-release versions with "dub upgrade --prerelease" if release versions are also available.

Don't upgrade unmanaged packages. Fixes #171. ...

Also renames Action.Type.get to fetch.

Adjust style of dub.internal.vibecompat.data.json to latest vibe.d and use vibe.data.json if available.

Move PlacementLocation to the proper module.

Fix up-to-date tracking of import and string import files.