resolve template values when getting checksums for extensions

[boegel]

With the fix, the enhanced test fails with:

ERROR: test_exts_list (test.framework.easyconfig.EasyConfigTest.test_exts_list)
Test handling of list of extensions.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Volumes/work/easybuild-framework/easybuild/tools/filetools.py", line 1305, in verify_checksum
    checksum = checksum[filename]
               ~~~~~~~~^^^^^^^^^^
KeyError: 'toy-0.0.eb'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Volumes/work/easybuild-framework/test/framework/easyconfig.py", line 512, in test_exts_list
    exts_sources = eb.collect_exts_file_info()
  File "/Volumes/work/easybuild-framework/easybuild/framework/easyblock.py", line 735, in collect_exts_file_info
    if verify_checksum(patch, checksum, computed_checksums[patch]):
       ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Volumes/work/easybuild-framework/easybuild/tools/filetools.py", line 1307, in verify_checksum
    raise EasyBuildError("Missing checksum for %s in %s", filename, checksum)
easybuild.tools.build_log.EasyBuildError: "Missing checksum for toy-0.0.eb in {'toy-0.%(version_minor)s.eb': '177b34bcdfa1abde96f30354848a01894ebc9c24913bc5145306cd30f78fc8ad'}"

This reproduces the problem reported in easybuilders/easybuild-easyconfigs#22090

edit: This is fallout from #4516, where templates are being resolved for sources in EasyBlock.collect_exts_file_info, but not in checksums.

[lexming]

LGTM

As discussed, templates in dict keys should be avoided. However, given that this has been behavior in EasyBuild for a long time, we need this change to not break existing easyconfigs.