Canonicalize Offset Names

[cancan101]

Reposted from: https://github.com/pydata/pandas/pull/5148/files#r10454189

Currently you get the following behavior because there is not a canonicalized offset name:

In [10]: get_offset("QS-JAN") == get_offset("QS")
Out[10]: True

In [29]: get_offset("QS-JAN").name == get_offset("QS").name
Out[29]: False

n [31]: get_offset("QS-JAN") is get_offset("QS")
Out[31]: False

[jreback]

on master

In [35]: get_offset("QS-JAN") == get_offset("QS")
Out[35]: True

In [36]: get_offset("QS-JAN").name == get_offset("QS").name
Out[36]: True

In [37]: get_offset("QS-JAN") is get_offset("QS")
Out[37]: False

this looks right. These are now new objects, not the same object, so they compare equal, but not identical. see #11192