Fix env duplication on merged-usr systems

[Elvis Pranskevichus (elprans)]

On modern (merged-usr) Linux systems, /bin, /sbin and /usr/sbin
are symlinks to /usr/bin. There is no point in reporting the same
Python installation four times, so canonicalize search paths before
searching.

Before:

Breakdown for finding Environments:
-----------------------------------
GlobalVirtualEnvs    : 17.412201ms
Locators             : 225.284494ms
Path                 : 433.162905ms
Workspaces           : 2.161556ms

Environments (41):
------------------
GlobalPaths          : 14
LinuxGlobal          : 16
VirtualEnvWrapper    : 11

After:

Breakdown for finding Environments:
-----------------------------------
GlobalVirtualEnvs    : 16.595382ms
Locators             : 223.759511ms
Path                 : 313.276036ms
Workspaces           : 1.418024ms

Environments (21):
------------------
GlobalPaths          : 2
LinuxGlobal          : 8
VirtualEnvWrapper    : 11

[Don Jayamanne (DonJayamanne)]

Elvis Pranskevichus (@elprans)
FYI, some test failures.

[Elvis Pranskevichus (elprans)]

Elvis Pranskevichus (@elprans) FYI, some test failures.

Don Jayamanne (@DonJayamanne), the verify_python_in_jupyter_contaner test fails because in that container /home/codespace/.python/current is a symlink to /usr/local/python/current which is itself a symlink to /usr/local/python/3.10.13. Canonicalization made it return the latter whereas the test expects the former two.

Technically, symlinked Python prefixes aren't real distinct installations (which is the point of this PR), but if it's important to return an uncanonicalized executable path for some reason, we can narrow the scope of this change to just canonicalize /bin, /usr/bin and friends.

[Elvis Pranskevichus (elprans)]

Don Jayamanne (@DonJayamanne), any feedback on the above? I've modified the tests to pass.