Make the template plugin example depend on the plugin as a real dependency

[amirh]

Description

The template for the example plugin app was adding the path dependency to the parent plugin as a dev_dependency.
When the plugin template was introduced in #9076 this was the case depending whether the example app was created with a driver test, #11304 made this the case unconditionally.

The example app builds with the plugin as either a real dependency or dev_dependency as the semantic difference between the two only applies to transitive dependencies (Pub Dev Dependencies). Though it still violates the convention where stuff that's imported from lib/ is a real dependency.

Tests

• Verify that the pubspec for the example app that's generated as part of the plugin template has a real dependency on the parent plugin.

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Breaking Change

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[jmagman]

Is there an issue tracking first-party plugin example projects getting this change? Would be nice to prove everything is working there as well.

[amirh]

@jmagman most first party plugins are including the plugin as a real dependency in the example app already, e.g: https://github.com/flutter/plugins/blob/9859a383c7d15966eb9165f2616afeb4ee53129a/packages/image_picker/example/pubspec.yaml#L9

[harryterkelsen]

LGTM