fix(venv): Don't use dirs as the interpreter

[arrdem]

Fixes #664

On Linux and MacOS, the current_exe() path has likely been realpath-ed by the system or the libc somewhere along the line. This means that if the user types .venv/bin/python3, the entire symlink chain that may point into will be chased and we'll get a path somewhere inside the Bazel execroot/ output tree. Critically on Linux this symlink tree goes past the .runfiles/ directory entry the user intended to create a symlink to, which prevents our interpreter shim from delegating to a .runfiles/ packaged Python like the user desired.

In order for us to resolve "just enough" symlinks to identify the .runfiles/ tree, we need to manually resolve the path of the current executable. Previously doing so would create a false positive if there was a ./python or ./python3 directory, and the interpreter shim was invoked as python3 which is ambiguous as to whether the user meant ./python3 or python3 from the $PATH.

The fix here centers on two insights:

• The "real" interpreter can never be a directory (directly fixes the bug)
• The "real" interpreter's absolute path must have the same absolute path as whatever current_exe() is

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Fixed a bug which would cause the interpreter shims to identify ./python3/ or ./python/ (directories) as the real path of the interpreter "python3" rather than falling back to which() behavior.

Test plan

• Manual testing

[github-actions]

e2e/use_release folder: LCOV of commit 545ee19 during CI #2084

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