DM-50999: Reorganize to support EUPS-less usage

[timj]

• Use eups: URI to find external config.
• Simplify config loading now that relative paths are handled automatically.
• Remove some getPackageDir usage.
• Allow config, policy, and corrections to appear as python package resources.

Requires lsst/obs_base#543 and lsst/obs_lsst_data#32

[timj]

It would be pretty easy to change makeCamera and IsrCalib.readtext to work with ResourcePath. Maybe I should do that.

[erykoff]

I think this is fine but I am still not sure why some places are eups: and some are resource:.
Also, have the config loads always been relative and somewhere along the way we cargo culted full paths?

[timj]

Configs did not work with relative paths until lsst/pex_config#131 where the code was "fixed" to make it work properly (otherwise it would try to load relative to cwd). Hence all the __file__ shenanigans (which replaced a load of getPackageDir calls). The fixes to pex_config make things much simpler.

[timj]

The eups:// has to be used when referring to a package that hasn't yet been converted to support resources directly. There is one config referring to a meas package.

resource:// can be used for packages that have been migrated. I see one eups pointing to obs_lsst_data that I did before I migrated obs_lsst_data so I can fix that one. It's more explicit to use resource:// if it works since it removes the guess work if the _DIR env var is not set.

[timj]

@erykoff I think at one point you asked about readText supporting ResourcePath. I think that is something we can ticket but there are lots of IsrCalib classes that would need to be modified to support it and they all have to be converted at once before it can be used.