[SR-5383] Display appropriate message when trying to build system module packages

[aciidgh]

Previous ID	SR-5383
Radar	rdar://problem/33057869
Original Reporter	@aciidb0mb3r
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Package Manager
Labels	Bug, StarterBug
Assignee	None
Priority	Medium

md5: 5327f05f11e6717ca8ca0e7444fa59e5

is duplicated by:

• SR-8522 System Package failed to build with ambiguous errors

relates to:

• SR-8645 BuildPlan error descriptions aren't printed

Issue Description:

Building a system module package emits this error:

<unknown>:0: error: unexpected 'commands' value (expected map)
<unknown>:0: error: unable to load build file
error: terminated(1): /Users/ankit/workspace/swift.org/swiftpm/.build/debug/swift-build-tool -f '.....build/debug.yaml'

We should not try to build such packages and display appropriate notice message.

[aciidgh]

#1696