deploy/retrieve need option to override sfdx-project.json's default path

[tggagne]

sfdx-project.json is (or should be) tracked in git repositories. For this reason, needless edits to the file should be avoided when a project includes multiple paths. Today, the only way for --metadata or --manifest options to work correctly is by editing the default path in sfdx-project.json. I propose sf cli have a mechanism to override the default with a command-line option.

If sf cli's project deploy and retrieve had an option to specify which path assets should be deployed from or retrieved-to, then developers wouldn't need to edit it when working in projects with multiple packages and/or paths.

--source-dir and --metadata-dir don't work because they can't be used when --metadata is also included on the command line. This is important because if many metadata types are being deployed (10-or-more aren't uncommon) then the deploy becomes long and difficult to read. Manifest files (package.xml) are also checked into git and we don't want to keep editing those.

The documentation explains which is the default path, but nowhere does it explain how sf cli may override it. This would be how it could be overridden without unnecessary edits.