Automatically detect source based on local context

[wagoodman]

What would you like to be added:
Derived from #558 (comment) it would be ideal to try and get more specific metadata for sources that would be directory scans, but could be more specific, such as a GitSource (and we capture things like the current git commit, tag, state, etc).

Why is this needed:
This would allow for the SBOM to capture more complete information for a source when the input is not detailed enough. For instance, if scanning . we don't want the source name to be . if this is a git source, instead we should report the url for the repo, such as github.com/anchore/syft.

The same can be said for other content in a directory: if the directory is a source directory for a python project, then pull in information from the pyproject.toml to get the name of the source.

This implies that this could allow for multiple metadata sources for a single input directive (e.g. scan . might resolve in a list of a GitSource and a PythonSource [made up]).