Canonicalize Offset Names
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
```
