Golang: Display Go external libraries based on importpath structure (bazelbuild/intellij PR import #2973)

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Golang: Display Go external libraries based on importpath structure (bazelbuild/intellij PR import #2973)

This is a modified version of the original change by Brandon Lico with a few modifications:

• read from the fileToImportPathMap directly when modifying the structure
• add unit tests.

Design choices:

• We don't inherit from BlazeExternalSyntheticLibrary since that contains irrelevant logic. Also we can't logically fulfill the "Only supports one instance per value of presentableText." condition that's outlined in the class javadoc since we could have nodes with identical presentableText properties in the tree in different positions.

Discussion thread for this change

Issue number: #1744 #2365

Description of this change

Modifies the structure of the external "Go Libraries" project view to be based on importpath rather than a flat list of files. See my comment within #2365 for an example of how the new structure looks.

Closes #2973

Tests

Unit tests added.