Verification Code and Checksum computation updates

[SamuraiAku]

• On further review on the SPDX specification, updated the algorithm for computing a package verification code
• Replaced the function spdxchecksum() with ComputePackageVerificationCode() and ComputeFileChecksum()
• Resolved #40: Handling of symbolic links when computing the package verification code
• Resolved #29: Support checksum calculation on a single file
• Resolved #28: Use the Logging standard library to record all the files processed and their checksums

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d9b3b69) 98.39% compared to head (d4132c9) 98.43%.

Files	Patch %	Lines
src/checksums.jl	97.56%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #43 +/- ## ========================================== + Coverage 98.39% 98.43% +0.03% ========================================== Files 17 17 Lines 810 830 +20 ========================================== + Hits 797 817 +20 Misses 13 13 ```

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