`dub run` will now automatically fetch a package if it's not found locally

Starting with this release, `dub run <mypackage>` makes sure that the package is available locally.
This means that now a `dub fetch <mypackage>` is no longer required and all a user of a library needs to run your dub package is `dub run`:

$(CONSOLE
> dub run gitcompatibledubpackage
gitcompatibledubpackage wasn't found locally, but it's available online:
---
Description: Example of a DUB package also usable as git submodule. For DUB test suite.
Version: 1.0.4
---
Do you want to fetch gitcompatibledubpackage? [Y/n]:
)

An optional `--yes` (`-y`) flag is provided for non-interactive use:

$(CONSOLE
> dub run --yes gitcompatibledubpackage
gitcompatibledubpackage wasn't found locally, but it's available online:
---
Description: Example of a DUB package also usable as git submodule. For DUB test suite.
Version: 1.0.4
---
Fetching gitcompatibledubpackage 1.0.4...
Building package gitcompatibledubpackage in /home/seb/.dub/packages/gitcompatibledubpackage-1.0.4/gitcompatibledubpackage/
Performing "debug" build using dmd for x86_64.
gitcompatibledubpackage 1.0.4: building configuration "exe"...
Linking...
Running ../../.dub/packages/gitcompatibledubpackage-1.0.4/gitcompatibledubpackage/gitcompatibledubpackage
Hello DUB
)

If one wants to run a specific version of a package, it can be passed to `dub run` too:

$(CONSOLE
> dub run gitcompatibledubpackage@1.0.3
Fetching gitcompatibledubpackage 1.0.3...
Building package gitcompatibledubpackage in /home/seb/.dub/packages/gitcompatibledubpackage-1.0.3/gitcompatibledubpackage/
Performing "debug" build using dmd for x86_64.
gitcompatibledubpackage 1.0.3: building configuration "exe"...
Linking...
Running ../../.dub/packages/gitcompatibledubpackage-1.0.3/gitcompatibledubpackage/gitcompatibledubpackage
Hello DUB
)