Ensure ESM compilation correctly detects file extension

[philippjfr]

When compiling a component pointing to an ESM file on disk it was incorrectly adding a leading period, leading to compile errors.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.25%. Comparing base (26c2e7a) to head (f89bcc3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/compile.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7446      +/-   ##
==========================================
- Coverage   82.25%   82.25%   -0.01%     
==========================================
  Files         337      337              
  Lines       50678    50680       +2     
==========================================
  Hits        41685    41685              
- Misses       8993     8995       +2     

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