Use code search as input for target repository list

[jamestelfer]

Follow up to #398, adding code search as another source (see initial discussion).

I propose adding a --code-search flag (GitHub only, initially) that would use the flag as an SCM-specific search query that would gather the set of target repositories by searching across code in the target system. The syntax accepted by the flag would be that supported by the underlying SCM, and would be passed through directly.

Example searches:

• partial names with boolean queries: org:buildkite-plugins language:shell docker
• one or multiple owners user:lindell filename:go.mod go "1.19"

Drawbacks to this approach include that mentioned in #398.

Relevant GitHub documentation:

• GitHub repository search API
• GitHub repo search qualifiers
• GitHub search query syntax (this is the legacy search, as the API does not yet use the new search engine).

[github-actions[bot]]

Included in release v0.49.0 🎉