This works with v0.123.8 and later (and maybe earlier too):
[[module.mounts]]
source = 'foo.md'
target = 'content/foo.md'
But remember to include the default mount:
[[module.mounts]]
source = 'content'
target = 'content'
[[module.mounts]]
source = 'foo.md'
target = 'content/foo.md'