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Fixes #6239
Fixes #7946
Fixes #8017
Fixes #8717
Closes #7987
Closes #8718

* Improve gradle plugin task parsing

Added _gradle and _gradlew as symbolic links to gradle.plugin.zsh,
otherwise the plugin was not properly loaded.

Output from `gradlew tasks --all` is now parsed in two levels,
first we find segments between `------...` and a newline.
Second, all those lines are parsed and cleaned using awk
and added to .gradletasknamecache.

Tested on gradle 2.13, and gradlew 2.14.

* Remove .gradletasknamecache before regenerating it

Remove the .gradletasknamecache file to avoid having an unnecessary
newline at the top of the file when regenerating it.

* Improve gradle task parsing by writing .gradletasknamecache atomically

Previously the .gradletasknamecache file was written line by line inside a parsing loop,
which could cause errors such as half-written cache files if the process was aborted.

This also removes the need of deleting the .gradletasknamecache file before parsing.