Fix sage --info <package>

[jdemeyer]

Doing

./sage --info mpir

should show the SPKG.txt of the mpir spkg.

Apply:

1. attachment: 12606-sage-info.patch to the SAGE_ROOT repository.
2. attachment: 12606_test_and_document_info.patch to the Sage library.

CC: @jhpalmieri

Component: scripts

Author: Jeroen Demeyer

Reviewer: John Palmieri

Merged: sage-5.4.beta0

Issue created by migration from https://trac.sagemath.org/ticket/12606

[jdemeyer]

Description changed:

--- 
+++ 
@@ -4,3 +4,7 @@
 ./sage --info mpir
 ```
 should show the `SPKG.txt` of the mpir spkg.
+
+**Apply**:
+1. [attachment: 12606-sage-info.patch](https://github.com/sagemath/sage-prod/files/10654999/12606-sage-info.patch.gz) to the `SAGE_ROOT` repository.
+2. [attachment: 12606_test_and_document_info.patch](https://github.com/sagemath/sage-prod/files/10654998/12606_test_and_document_info.patch.gz) to the Sage library.

[jdemeyer]

Author: Jeroen Demeyer

[jdemeyer]

Attachment: 12606_test_and_document_info.patch.gz

[jhpalmieri]

Reviewer: John Palmieri

[jhpalmieri]

comment:2

Attachment: 12606-sage-info.patch.gz

Looks good to me.