clean up internal imports

[mlin]

Make all of the WDL package's internal imports relative.

Avoid abbreviating imported module names, to improve readability.

[coveralls]

Pull Request Test Coverage Report for Build 786

• 672 of 691 (97.25%) changed or added relevant lines in 11 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 96.903%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Expr.py	104	105	99.05%
WDL/Tree.py	71	72	98.61%
WDL/StdLib.py	147	151	97.35%
WDL/runtime/task.py	65	69	94.2%
WDL/_parser.py	77	86	89.53%

Files with Coverage Reduction	New Missed Lines	%
WDL/StdLib.py	4	93.36%

Totals
Change from base Build 781:	-0.02%
Covered Lines:	3567
Relevant Lines:	3681

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💛 - Coveralls