Testsuite: Replace --version=x.y.z with package@x.y.z

testsuite: Use built dub, not system one in more tests ...

Some places were missed during the last fixups.

Testsuite: Fix calls to die to always include line numbers ...

'die' actually taking two parameters seems to be a little-known fact.

Make 'dub remove' remove all packages by default in non-interactive mode ...

Given how often it is used in the test-suite, it should be self-obvious
that it will be what most users want.

Support fetch sub-package