implement `buckaroo explore <url> [constraint]` command

[nikhedonia]

buckaroo explore <url> should show to the user all valid versions of <url>

Output could look like the following:

buckaroo explore github.com/org/package
{branch=develop} -> rev=fc08c1fe28 (cached)
{branch=master, tag=latest, 1.67.0} -> rev=9e9a813c8

Most of the logic can already be found in the Source Explorer.

Exploration of transitive dependencies may be a separate feature