allow assigning of categories using package name

[smuzaffar]

This change should allow bot to use cmssw package names to assign categories to an issue e.g. following comments should allow bot to properly assign L2 categories to an issue

assign|unassign Package/Name[,Package/Name[...]]
assign|unassign from Package/Name[,Package/Name[...]]
assign|unassign package Package/Name[,Package/Name[...]]

@Dr15Jones requested this feature and I agree that it is very useful (avoid doing a lookup in https://github.com/cms-sw/cms-bot/blob/master/categories_map.py to find the package/category mapping)

[cmsbuild]

A new Pull Request was created by @smuzaffar (Malik Shahzad Muzaffar) for branch master.

@cmsbuild, @smuzaffar, @aandvalenzuela, @iarspider can you please review it and eventually sign? Thanks.
@rappoccio, @sextonkennedy, @antoniovilela you are the release manager for this.
cms-bot commands are listed here

[smuzaffar]

+externals

local tests look good for cms-sw/cmssw#43009 (comment)

> ./process-pull-request -n 43009
...
==> assign CondCore/CondDB,MagneticField/Interpolation,DataFormats/Common assign ['db', 'reconstruction', 'core']
==> unassign DataFormats/Common unassign ['core']
...

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs after it passes the integration tests. This pull request will now be reviewed by the release team before it's merged. @rappoccio, @antoniovilela, @sextonkennedy (and backports should be raised in the release meeting by the corresponding L2)