Hide New Package menu unless package selected

[danyeaw]

Fixes #1952 which is an assertion error while creating a package that is nested under a non-package item.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and I am following the Contributor guide
• I have read, and I understand the Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

The New Package option was always visible on the right click menu.

Issue Number: #1952

What is the new behavior?

The New Package option is disabled unless you are right-clicking on a package.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.27%.

Quality metrics	Before	After	Change
Complexity	2.90 ⭐	3.05 ⭐	0.15 👎
Method Length	75.71 🙂	76.00 🙂	0.29 👎
Working memory	6.80 🙂	6.80 🙂	0.00
Quality	73.50% 🙂	73.23% 🙂	-0.27% 👎

Other metrics	Before	After	Change
Lines	325	326	1

Changed files	Quality Before	Quality After	Quality Change
gaphor/ui/namespace.py	73.50% 🙂	73.23% 🙂	-0.27% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
gaphor/ui/namespace.py	popup_model	9 🙂	213 ⛔	8 🙂	52.95% 🙂	Try splitting into smaller methods
gaphor/ui/namespace.py	Namespace.open	4 ⭐	187 😞	8 🙂	59.72% 🙂	Try splitting into smaller methods
gaphor/ui/namespace.py	Namespace._on_show_popup	3 ⭐	127 😞	10 😞	62.53% 🙂	Try splitting into smaller methods. Extract out complex expressions

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[amolenaar]

Yup. Sometimes a solution is as simple as this :).