Fix #817 Allow specifying global extension order

[tburny]

Allow specifying the order of global extensions explicitly.

For instance if

• spock-spring provides the SpringExtension via its IGlobalExtension file,
• MyProject provides com.example.MyExtension
• and MyExtension depends on SpringExtension
  then the MyProject/src/main/resources/META-INF/services/org.spockframework.runtime.extension.IGlobalExtension would look like this:

org.spockframework.spring.SpringExtension
com.example.MyExtension

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This change is 

[codecov]

Codecov Report

Merging #818 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #818   +/-   ##
=========================================
  Coverage     74.45%   74.45%           
  Complexity     3327     3327           
=========================================
  Files           359      359           
  Lines         10355    10355           
  Branches       1292     1292           
=========================================
  Hits           7710     7710           
  Misses         2177     2177           
  Partials        468      468

Impacted Files	Coverage Δ	Complexity Δ
...pockframework/runtime/GlobalExtensionRegistry.java	90.9% <100%> (ø)	24 <0> (ø)	⬇️

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[leonard84]

Spock doesn't allow duplicate global extensions on the classpath see #738, this approach also doesn't deal with cycles and won't work with Java 9 modules.