track and resolve symlinks when reading context timestamps/hashes

[sokra]

refactor context snapshotting for abstraction purposes
add (symlink) unit test cases for FileSystemInfo snapshotting

fixes #12246

This allows symlinks to be correctly snapshotted by FileSystemInfo.

• circular references are eliminated
• context that includes a symlinked directory will include content of that directory
• For all snapshotting modes (timestamps, hashes, or both)

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

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