environment views: single pass view generation (#29443)

Reduces the number of stat calls to a bare minimum:
- Single pass over src prefixes
- Handle projection clashes in memory

Symlinked directories in the src prefixes are now conditionally
transformed into directories with symlinks in the dst dir. Notably
`intel-mkl`, `cuda` and `qt` has top-level symlinked directories that
previously resulted in empty directories in the view. We now avoid
cycles and possible exponential blowup by only expanding symlinks that:
- point to dirs deeper in the folder structure;
- are a fixed depth of 2.

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