Refactor source creation and add --from flag

[kzantow]

This PR makes the following high-level changes:

• The source.Detect function has been removed, instead use:
• the syft.GetSource function, which has been added as the entrypoint to use syft defined sources
• 3 source types have been moved to their respective packages:
  • source/filesource
  • source/directorysource
  • source/stereoscopesource
• A --from flag is added, allowing the user to select the source(s) to use
• An exported source/sourceproviders package has been added which defines the set of tagged providers Syft implements, available by calling the the All function

Closes #1783

TODO:

• determine if/what replacement test should be implemented for syft/source/detection_test.go, as there isn't the same need to test the "detection" aspect, but there probably are still some valid test cases here
• revisit error handling from sources e.g. should we display each file not found with potentially more info?
• is the testutil.Chdir(t, "..") acceptable? (at least as part of this PR to limit the changes)
• correct behavior for generateSBOMForAttestation?

[wagoodman]

I think some of the source/* packages should be a little more consistent:

• filesource: good as is
• directory -> directorysource
• stereoscope -> stereoscopesource

[wagoodman]

great refactor! 💯