Allow package as an imperative verb in D401 (#356)

If a docstring starts with "Package" it's probably something like:

```python
"""Package the files into a tarball."""
```

So don't flag it in D401.

Co-authored-by: Sambhav Kothari <sambhavs.email@gmail.com>

Author: gibfahn

docs/release_notes.rst

@@ -16,14 +16,17 @@ New Features
 
 * Add flag to disable `# noqa` comment processing in API (#485).
 
+Bug Fixes
+
+* Treat "package" as an imperative verb for D401 (#356).
+
 5.1.1 - August 29th, 2020
 ---------------------------
 
 Bug Fixes
 
 * Fix ``IndexError`` crash on one-line backslashed docstrings (#506).
 
-
 5.1.0 - August 22nd, 2020
 ---------------------------
 
@@ -43,7 +46,6 @@ Bug Fixes
 * Support backslash-continued descriptions in docstrings (#472).
 * Correctly detect publicity of modules inside directories (#470, #494).
 
-
 5.0.2 - January 8th, 2020
 ---------------------------
 
@@ -122,7 +124,6 @@ Bug Fixes
 
 * Fix parsing tuple syntax ``__all__`` (#355, #352).
 
-
 3.0.0 - October 14th, 2018
 --------------------------
 

src/pydocstyle/data/imperatives.txt

@@ -137,6 +137,7 @@ open
 output
 override
 overwrite
+package
 pad
 parse
 partial

src/pydocstyle/data/imperatives_blacklist.txt

@@ -63,7 +63,6 @@ module
 new
 number
 optional
-package
 placeholder
 reference
 result