Support alias without : in URL

Summary

As part of adding Perl5 mirroring we ran into a special case where the normal usage of alias:repo just doesn't work because the repo name is different for main and mirror. Different variations were trued including just alias. The only option which Buildstream seemed to accept was "alias:" so repo is treated as empty string. This seems silly, we should be able to write a bare alias as url. freedesktop-sdk/freedesktop-sdk!608 (merged)

Steps to reproduce

Remove the : from FD.o elements/base/perl.bst and try to build it

What is the current bug behavior?

Buildstream doesn't resolve the alias but tries to clone a url "perl5"

What is the expected correct behavior?

Alias is resolved

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2.3

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